I upgraded to PowerCLI 5.1 Release 1 build 793510 and it seems to be working fine now. I'll just have to make sure whoever runs my scripts knows to be on this build or later... otherwise the behavior will change.
FreeSpaceMB : 238941
CapacityMB : 511744
CapacityGB : 499.75
FreeSpaceGB : 233.3408203125