Beta 1 doesn't seem to support this yet. Its easily possible by using the new Taskbar APIs and specifically the custom categories feature. I think their main problem might be that the solution files are associated with an app called VSLauncher.exe (this small app determines what version of VS to load when you launch a SLN file).
If you want to "pin" your fav solution files to any version of VS, one option this is (I currently used this with VS 2008 and 2010 Beta 1:
- Open Explorer and browse to any SLN file you want to pin.
- Drag it and pin on the taskbar (it will pin to VSLauncher.exe; this is okay)
- Now just repeat the steps for other SLN files.
When you click on a SLN file from the above pinned program's jumplist, another instance of VS2010/VS2008 will launch. It won't show your recent projects or solutions, but atleast it allows you to pin your fav/recent solutions manually. I think you can use this for CSProj or other project files as well.