I do sympathise: only only category per item must be a real constraint. (Or discipline!)
As my big nasty multinational employer has lent me a pocketPC, I don't have this issue.
At the moment, I use a list manager to hold my (rather scanty) projects. (I had ANOTHER attempt at a weekly review and shocked myself with just how little I am even trying to drive as a project.) Maybe I DO need to link tasks to projects better: perhaps I'll try using the categories list as a working projects list...