Lately some discussions came up about the compatibility of the upcoming VirtueMart 1.1 and Joomla! 1.0 series, so let's clear things up.
Virtuemart 1.1 will support
- Joomla! 1.0,
- Joomla! 1.5 (natively),
- Mambo 4.6.3
- and probably the upcoming Mambo 4.7.
So there's no need to upgrade to Joomla! 1.5 if you don't want to. You can find more information about VirtueMart Compatibility in the Developer Guidelines on the Wiki.
Update: VirtueMart 1.1 will support Joomla! 1.5 natively. The current SVN code runs on Joomla! 1.5 with the Legacy Plugin unpublished.