I think if anything forces Clegg out it'll be the other elections. They're expected to lose over a third of the council seats they're defending in England, a particularly bad showing in Scotland (a couple of polls even put them in 5th place behind the Greens) and possibly (though not particularly likely) wipeout in Wales.

I doubt it'll destroy the Lib Dems, although the party could change beyond recognition, including a possible Bavarian-style split (the party in Scotland splitting from England & Wales), and there may well be a leadership challenge. I doubt it'll end the coalition (it would be career suicide), but it could mean it's more likely the next election would be around 2013 rather than 2015.