I've been reading this thread, and I thought of a way you guys could deal with DLC.
For this example, I'll be using Borderlands for PC (this would also apply to XBox 360 and PS3.)
The idea is that Borderlands would be added as a regular entry, but DLC packs would be added as a sub-entry for the game. So, let's say someone adds the main Borderlands game to their collection. Once that is done, a window could be shown on the screen, which says "This game has these DLC packs available. Would you like to add the ones you own?" The user would then choose to do so or leave it for later. If someone searches for a particular DLC on the site, the normal stats would be shown, along with info saying that this item is an expansion of X game (with the appropriate cover artwork if available.) Ideally, a user -would not be allowed- to add these DLC packs to their collection if they don't own the base game.
Also, a user could be given the option to count DLC as single game entries or to just ignore them in their total game count.
Note that to me, a "valid" DLC entry would be something that gives a substantial addition to the game. Character packs may count... but music game song packs would not, I suppose.)