See Mark/Space, Inc. - Support
which is a Mark/Space KB article on syncing with Missing Sync and BES. I'm not sure if you would have to turn things on/off if you just decided you wanted to sync only with your Mac for notes and tasks. So, hopefully, you should be able to just leave it to sync cal/contacts wirelessly as I think the KB article is focussed on if you want to sync with both
a Mac and BES. However, not being a BES user myself I can't say for certain.
Since you're a GTD user there might be a few wrinkles which annoy you. I too am a GTD'er. I use NextAction! on the BB. The latest version has project support, but it does this through categories. The problem is that the way you sync BB-->iCal-->Entourage doesn't support categories. Actually, I don't bother with Entourage for Tasks and use iGTD which is pretty good. Of course, that handles projects in a different way (by appending p:Project_Name to the task title) so projects in NextAction! don't work. NextAction! is still pretty useful for me, though.
For memos/notes, I don't bother with Entourage, but sync to the Mark/Space Notebook that you get with Missing Sync. Actually, this is quite a decent app.