Softimage saves the location of the license server in the registry. On Vista, the location is saved in the ADSKFLEX_LICENSE_FILE value:
HKEY_CURRENT_USER\Software\FLEXlm License Manager
On my XP system, the value is saved under HKEY_LOCAL_MACHINE.
Over time, you may end up with some obsolete license server locations saved in the registry. For example, if you replace your license server, the location of the old server is still in the registry.
So whenever you cannot get a license, and everything else seems to check out ok, open up regedit and clear the ADSKFLEX_LICENSE_FILE value, to make sure Softimage is connecting to the right license server.
I had a support case the other week where the customer wrote
xsibatch cannot get a license, even though setenv.bat an exact copy of the setenv.bat from a machine where xsibatch can get a license.
It turned out the problem was because of ADSKFLEX_LICENSE_FILE. After he deleted that value, xsibatch connected to the license server specified by setenv.bat and got a license.