I think Disk Utility on the 10.5 DVD can remove the bootcamp partition and expand the OSX partition. It does carry risks though.
I would install the 10.5, do a Time Machine backup, then restore that backup from the install DVD into a brand new full partition or resize it from 10.5.
Wirelessly posted (iPhone 16GB: Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420.1 (KHTML, like Gecko) Version/3.0 Mobile/4A102 Safari/419.3)
have you tried setting your clock back to before it expired?