Sounds like your players aren't really interested in your plot (they'll say they are if you ask them, but that's probably because they want to keep you happy so you'll keep running the game and they don't have to). My guess is that they're probably there to 1) socialize and 2) "play the game" (which means, essentially, to kill stuff and harvest treasure and XP, and perhaps solve an occasional logic-puzzle) and everything else is at best secondary to and at worst a distraction from those two goals. The players are there to have fun, and if their idea of fun is socializing and killing things and taking their stuff, that's what they're going to want to do, whether your game provides for it or not.
The best solution with this type of players IMO is to roll with it and adapt your plot to their shallow short-attention-span playstyle -- throw out your house-rules, radically simplify your plots, and always keep the action moving at a brisk clip (if the characters are in pretty much constant danger of dying horribly right now the players will be less likely to get distracted -- there will still probably be lots of crosstalk and OOC chatter (since that's, probably, part of what they're there for -- if they didn't want to socialize they could play CRPGs at home) but it will tend to be more about the action in-game ("oh, man, your character is so going to get killed by that thing!" "duede! get your lazy butt over here and help me out!" etc.) rather than out-of-game stuff, which is what you want). Don't be afraid to spoonfeed the plot to them or even blatantly railroad them, so long as it keeps the action moving and the excitement level high (for example, if your intended plot was to involve the players spending several sessions investigating in town to gather clues to figure out who the bug bad guy is and where his hideout is, and then traveling across the wilderness to that place to raid it, consider skipping (or at least radically shortening) all of that investigation and travel and cutting right to the climax -- the action-heavy raid on the bad guy's lair).
Which isn't to say you should throw out your plots entirely and face the party off against random opponents and challenges by any means, because even casual players want to feel like they're accomplishing something, like they're fighting these bad guys and exploring this dungeon for a reason, and they want to build up histories (of a sort) for their characters. As long as you keep the plots simple and the action moving, you'll probably find the players becoming engaged in the plot (albeit probably in a shallower way than you might have hoped for -- they might not remember the bad guy's name or what exactly he did that was so bad, but they'll remember that they hate him and want to kill him and take his stuff!) and that should provide at least some satisfaction to you -- better they appreciate a stripped-down/dumbed-down version of your plot than fail to appreciate the full, elaborate version, right?
ETA: And I agree with fusangite
![:]](http://www.enworld.org/forum/images/smilies/devious.png "Devious :]")
