Jesse Hollington recently wrote a great post called Getting more Focused with OmniFocus. He's totally got the Grouping Tasks by Session down, complete with a really nice hotkey/script.
The script takes a link in a task and executes it. So you can have a link to a perspective, application, URL, etc and go straight there with a key command. He uses Fastscripts to set the hotkey. I don't own that software, so I linked it up using Keyboard Maestro. Here's my setup:
Addendum: I'm now using (Option-Command-Space) as the key command. That may seem minor, but it isn't. Option-Command-Space and the completion command of Space are similar to each other. The "language" of engaging the keyboard are better paralleled this way.
