This morning virtuemart.net was successfully upgraded from Mambo 4.5.2.3 to Joomla!. The upgrade procedure was really easy and quick! It was just uploading the new Joomla files into the mambo root directory and running a small SQL upgrade script. I had decided to upgrade this site not only because of our support for the Joomla! core team, but also because of the slow and unmotivated support from the "new" Mambo core team after a security hole had been found. No newsletter from them, just a small patch (which seems to me quite unofficially released).
If you think about upgrading your Mambo site to Joomla! - just do it. You find upgrade instructions here. When you are running the "Mambo eCommerce Edition" with mambo-phpShop, you will first have to migrate to Joomla and after that install VirtueMart (don't think too complicated: it's no upgrade, but you just install VirtueMart and it uses the mambo-phpShop data). But beware of the unix permission system. If you can't delete or overwrite a file from ftp, you can use the joomlaXplorer component (which is for Mambo and Joomla!).
