Since VirtueMart 1.1 had been released, we were already working on the next version, which will be VirtueMart 1.2. This version will probably work with the same architecture than VirtueMart 1.1 and it will be compatible to Joomla! 1.0 and 1.5.
One of the bigger changes is a Plugin System, similar to the one in Joomla!. It will be possible to register plugins and use their functionality whenever certain events are triggered.
All payment and shipping modules have been rewritten and turned into VirtueMart Plugins. The API for the shipping modules has been changed, the structure of payment and shipping modules too. As soon as I find the time, I will create an end-user version of the developer manual for VirtueMart 1.2, so you can read what you need to change for your module to make it compatible for VirtueMart 1.2.
VirtueMart 1.2 will ship with an extension installer similar to the one used in Joomla! 1.5. It will be possible to install/uninstall extensions like payment or shipping plugins with one mouse click.
You can already test-drive a developer version of VirtueMart 1.2 (nightly build), which can be found on dev.virtuemart.net.