Having hosed a Gentoo guest on a VMware ESXi host by filling the partition (which VMware really doesn’t like) then attempting to fix it by mounting the partition in anther guest and fsck’ing it first, I got the error message “the parent virtual disk has been modified since the child was created” when I tried to boot the original Gentoo guest.
Googling pointed me to a nice post at Recovering VMware snapshot after parent changed.
Step two lists the following caveat:

“Look at the size of the snapshot virtual hard disk. If it is more than 2GB and you’re running a 32-bit OS, or it is more than the amount of memory that you have available, the following method will probably not work. You’re welcome to try though.”

I found this wasn’t an issue as it appears (at least as of ESXi 4.x) VMware has separated the vmdk “header” and “data”, putting the “header” in the “hostname.vmdk” file and the actual data in “hostname-flat.vmdk”. The original vmdk is now only a couple of hundred bytes and easily edited in vi. Grabbing the CID from the Gentoo.vmdk and modifying parentCID in Gentoo000001.vmdk had me back up and running (at least to the point that I could now boot the Gentoo guest, using an Ubuntu ISO so I could access the file system and clean it up. I moved /home to a new partition, fixing the space issue).
Next time, I’ll just be smart and build all systems with LVM, then I can just add more physical extents when I need more space.

Running Dog Leaugue has a good write up on how to install VMWare ESXi on a thumb drive.
With this I was able to get it up and running on a Dell PowerEdge 850 that would NOT install ESXi from a CD (couldn’t find a storage device to install to).