It's only a smudge if you can wipe it off with a cloth.  More likely what you're seeing is disc rot or disc clouding.  Disc rot will eventually render it unplayable as it means there's a break in the seal that holds the disc together.  Disc clouding on the other hand is just caused by an uneven distribution of the bonding glue that over time will cause milky swirls and clouding to be visible but usually don't effect the playback.  I have a lot of older discs that have the clouding going on, but haven't run into anything that stopped playing ::knock on wood::

Generally try your local DVD-rental-store, they usually have professional cleaning equipment that even can repair scratches (not if the DVD is damaged down to the aluminium) and mostly are willing to clean your DVD for a little fee.

In this special case I would say that Vega is possibly right. The only possibility next to rebuy the movie is to try to get an exchange because of a manufacturing fault

Disc rot, previously referred to as Laser Rot (for Laserdiscs), in my experience with DVDs is exceedingly rare, as compared to Laser Rot. Laser rot presented itself in a very different way, but the description of is pretty accurate and if you have a disc with rot there is no fix, just replace it.

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as per http://www.allformp3.com/dvd-software.htm information site -

"<Before DVDs there were laserdiscs (see 2.6 ), which were occasionally subject to what was commonly called laser rot : the deterioration of the aluminum layer due to oxidation or other chemical change. This usually results from the use of insufficiently pure metal for the reflective coating created during replication, but can be exacerbated by mechanical shear stress due to bending, warping or thermal cycles (the large size of laserdiscs makes them flexible, so that movement along the bond between layers can break the seal -- this is called delamination ). Deterioration of the data layer can be caused by chemical contaminants or gases in the glue, or by moisture that penetrates the plastic substrate.

Like laserdiscs, DVDs are made of two platters glued together, but DVDs are more rigid and use newer adhesives. DVDs are molded from polycarbonate, which absorbs about ten times less moisture than the slightly hygroscopic acrylic (PMMA) used for laserdiscs.

DVDs can have delamination problems, partly because some cases or players hold too tightly to the hub. Delamination by itself can cause problems (because the data layer is no longer at the correct distance from the surface) and can also lead to oxidation. Delamination may appear as concentric rings or a "stain" around the hub.

So far DVDs have had few "DVD rot" problems. There have been reports of a few discs going bad, possibly due to delamination, contaminated adhesive, chemical reactions, or oxidation of the reflective layer (see mindspring.com/~yerington/ and www.andraste.org/discfault/discfault.htm ). The most likely explanation for DVD deterioration is that during the early days of DVD (1997-2000), disc manufacturing processes and materials were not as good as they should have been. Many improvements have been made since then, so the minuscule problem has probably become even more minuscule.

There are also occasional reports of "cloudiness" or "milkiness" in DVDs, which can be caused by improper replication. An example is when the molten plastic cools off too fast or isn't under enough pressure to completely fill all the bumps in the mold (see this archived article from TapeDisc Business  for more). Minimal clouding doesn't hurt playback and doesn't seem to deteriorate. If you can see something with your naked eye it is probably not oxidation or other deterioration.

The result of deterioration is that a disc which played perfectly when it was new develops problems later, such as skipping, freezing, or picture breakup. If a disc seems to go bad, make sure it's not dirty, scratched, or warped (see 1.39 ). Try cleaning it and try playing it in other players. If the disc consistently has problems, it may have deteriorated. If so, there's nothing you can do to fix it, so you should try to get a replacement from the supplier.>"

I for one have not seen any evidence of DVD rot amongst my few titles.. But perhaps we should be asking if any of us here at Invelos has evidence and proof of such ??

I'm waiting for disc rot to enter my dvd life.
I've heard the ones most at risk are old Warner Bros. dual layer discs, including the original "Devil's Advocate" with the sculpture. I have that and recall a thread on the trade/sale forum here about some having unplayable copies of that already. I haven't played mine in a few months, but it was still working then (I could kinda see the layers seperating near the edges too though.
I have the whole disc V.O.B. files backed up on my external HD for that sad say when the disc does die, I'm not buying an edited re-issue or another rotted old one.

As for laserdisc, I started on that after dvd (back when there were hundreds of titles and SE's not on dvd yet, it's down to dozens now). In my small collection I already noticed rot issues with a couple, so far just the snowy speckling. I haven't spun any of those in about 2 years now though, and now have everything on dvd anyway (most legit, some customs).

I also have a $10 cleaning kit which has worked great for most smudges and light scuffs. Any deep scratch is beyond it, but it has saved a couple dozen ex-rentals I've bought. It's an all in one thing where you place a disc inside, drop a few drops of cleaning solution on the playing surface, close it and spin a rotating sponge around it (manually). 30 seconds or so will save what's savable, any more hasn't helped those few real problem discs I've had.