Actually we'll be adding support for Visual Studio 2005 in the next version that will be out in July. SSMS is based on VS 2005 so exposes many of the same VSIP interfaces although due to some seemingly minor omissions plugging into it is less straightforward than VS. On the other hand Visual Studio presents its own complexities since it uses a different SQL editor, so for example it's not as easy to get at the database connection properties as it is in SSMS or Query Analyzer.
We are still hoping to get an early access release of the new version of SQL Prompt out within the next week or so, however we're having installer issues at the moment due to bundling all of our installers into one uber-installer so it's possible that may not happen. If that's the case the first publicly available release will be the RC, which should be available in the middle of July.
The reason we didn't put Visual Studio support into the 3.0/3.1 releases was simply that our market research in the run-up to kicking off the 3.0 project indicated that 90% of people used SSMS or Query Analyzer as their primary editor, and VS 2005 came in with about 5% of the vote. Subsequently we've had a lot of people ask for the VS support, which isn't surprising given that we ran our survey over a year ago so you'd expect many more people to have adopted VS 2005 by now, so we've added it in this new release.
I should also point out that this will be a free upgrade for anyone who's already purchased SQL Prompt 3.0/3.1.
Hope that helps.