One of the biggest complaints we had on the February CTP of VSTO v3 was that you couldn't run your VSTO 2005 customizations in Office 2007. I'm happy to say we fixed that problem in the latest CTP for June.
What this means is that when you get the VSTO v3 June CTP your old customizations will continue to run in Office 2007. It does not mean there won't be any problems, though. It is a CTP, and there are likely issues. We haven't done extensive testing in this area yet, but we've recieved anecdotal evidence the things seem to run OK. Of course, I can't make any promises on pre-release software, as I'm sure you know. The reason we do these things is to get as much testing as possible and to actually find the problems, so please let us know where you have problems.
I also want to set realistic expectations. We've tried to make the runtime work, but we haven't gotten to the design time yet. This means that your VSTO 2005 projects will not work in Visual Studio once Office 2007 is on the machine. So don't put this on your main development machine. As with any Customer Technical Preview, we recommend you install it on a secondary machine that can be wiped. I use Virtual PC for this, it works well.
We wanted to get early Office 12 support out there as soon as possible, that was the main goal here. We also did not want to break existing customizations at runtime. That's hopefully fixed now too. The next goal is to make the design-time happier. There are many issues around different versions of Office on the same machine and how that interacts with VSTO projects. I'll post more on the design-time later.