I just fiddled this out after I accidentally deleted the content of my /boot partition on a UBuntu Jaunty system that I installed with an encrypted LVM root and home volumes. Although the problem occured just with a play and testing system which I had a full backup off, and could have easily recovered or reinstalled, I was curious about how to solve this problem - just in case it will happen to me some day when I can not just throw away the inaccessible system (whichg now, that I know how to solve it, will never happen ;) ).

It turned out to be a bit tricky.