It's really complicated problem that requires a lot of assignment related code to be altered. Because of that, we had to postpone it for a release that will introduce more changes to assignment filters (next big release, after one with Invoicing and SVN module). Sorry :(
Please check release notes article for full list of changes.
Installation and upgrade instructions are also available.
PS: If you are using activeCollab 2.2 or 2.2.1 beta, we have similar bug fix release coming in the next couple of days so stay tuned.