...Is it even possible to do that? I mean I've never tried, but I've never thought about trying either.
You can do it, but there's upper limits which are more restrictive than they used to be, on the PS3 anyway. It's due to the fact that they (supposedly) cap you to 6 devices total, 2 per device type (PS3/PSP/Vita) although in testing, we haven't actually hard hit the limit yet. (Mostly due to security. I'll find a third PS3 to try it on later)
The text proved very, very interesting too - as long as the content isn't locked per account requiring you to be logged into an account linked to that particular PSN account (When you check the information, it will tell you if this is the case) you can even play its multiplayer aspects even though only one of you paid for the game. Warriors Orochi 3 (PSN download) was one of the titles we used to test this, and we had the original account holder, and someone who downloaded the game using the account holder to download the game play each other, on different accounts.
Yeah, you'd THINK they'd locked that. Apparently not.
There's apparently issues with doing that a LOT (ie. Having it go around 20 friends) though, since you'll supposedly have to revoke past the second, and you can only do that a limited number of times, with a timer reset.
The Xbox 360 is the easier to work with though - you tie the DLC to one X360 (namely the one you bought it on), as well as tie it to an account (Namely the account you bought it on) - and that account can go virtually ANYWHERE, and be used as a 'loan account', no hopping issues.
It proves to me more practical that way, since you can throw your profile (with all its permissions) on a USB (or even zip it up, if you like), then take it to a friend's X360 and then access your DLC from the X360, regardless if they own it or not.
It's surprisingly useful if you want to take it to a convention, for example.