I agree with mcowger. The disk format doesn't change with snapshot consolidation. As soon as a virtual disk has a snapshot, it shows up as "Thick" in the GUI, because the GUI only checks the current virtual disk file for the ddb.thinProvisioned entry, which is only set to true in the base disk.
Do you have any older vmware*.log files from the time before you consolidated the snapshots? Please attach them to a reply post to see whether they explain what happened.
André