I would suggest if you plan to use mod_rewrite at any point in time you make sure the system degrades gracefully without it. I started writing a system awhile back where I unabashedly abused the hell out of mod_rewrite partly because I had just learned it, and partly because I was having too much fun making my urls far too humanly readable (don't ask me why that was fun, I don't know), until I ran into the roadblock of uploading it to my live server and finding out mod_rewrite isn't as prevalent in the hosting world as one thinks it should be.

In addition to VI's list there, which is a pretty good start, I wonder if maybe the project shouldn't consider starting from the ground up incorporating something such as ADODB for a database abstraction (or if the project members feel frisky enough, writing one of their own). This would open up the audience of this product as wide as possible.
-m