They won't walk away and I fully expect yet another u-turn from Camoron, especially when you take a look at the maths:
There are 23 LibDem ministers and if a minister votes against the government, he is expected to resign (before he gets sacked). Three of the ministers are peers, so there are 20 LibDems that either forego their ministerial privileges (and salaries), vote with the government (that wouldn't be popular with their constituency parties) or the simple solution is - they abstain.
So let's look at the maths now:
646 MPs (discounting the speaker & deputies)
305 Conservative
57 LibDems
254 Labour
30 Others
Of the 3 others, 8 are from the DUP, who usually side with the tories
So 305 (con) + 8 (dup) = 313
37 (libdem) + 254 (lab) + 22 (others) = 313
That's assuming that all Conservatives vote with the government, certainly not a cast-iron guarantee, given that some will be in danger of deselection due to merged constituencies. It also assumes that no LibDem minister has the balls to vote against and forego his ministerial bonus, in favour of keeping his constituents happy.
I have also not included the probable Labour gain in Corby, now Mensch has decided to sling her hook
It all becomes too close to call and rather than risk a defeat, I reckon Camoron will suddenly find that there won't be sufficient time in this parliament to debate the issue.