The thing that should worry the LibDems is Labour actually making itself a big tent centre-to-centre-left party which people actually want to vote for. Let's be honest, there's no appetite for Ed for PM at the moment.
Such a situation would also mean that the remnants of New Labour could be left out in the cold as the not-New-or-Old-Labour types realise they don't really need to win centrist-to-centre-right voters to win. Then again, an SNP surge or Scotland actually seceeding would torpedo this.
I would be shocked if there wasn't atleast one major attempt to get rid of Ed in 2012 (by who, I don't know, probably by Yvette), but then again, i'd actually be more shocked if he actually went. The rules to trigger a leadership ballot make it near impossible, unless he jumps himself.
I can't really see that a civil war over the leadership is going to do Labour any good.
My feeling is that Ed M's personal political positions are relatively attractive to left-leaning ex-LD voters, compared with those of most of the alternatives. (Certainly they are to me.) I'm not saying he's a perfect leader for Labour, because he obviously isn't, but it's not at all clear to me that a perfect leader for Labour actually exists.