It's nice to hear that you're interested in helping. If you need any specific help, don't hesitate to contact me (my email is in the about box)
1) Brian and I add code every now and then. I haven't touched it since the .6 release, and I don't know if Brian has or not.
2) Yeah, just send the diffs, or the full files, or anything else you want
If you'd like to actively contribute code, I'll add you to the project and give you CVS access.
3) Yes, there's probably a lot of weird specifics you'll need to know. The problem is, I can't tell you exactly what they'll be (everyone learns differently). So, when you run into something weird, don't hesitate to ask...it's probably something Brian or I have run across before.
4) That depends on what you want to get out of it. If I were to start on it, the first thing I'd change would be the current vector dependancy. It's slow, and (afaik) is the reason for the current 1024 bookmark limitation. Changing this to a linked list or a tree should fix things. If you're looking to add features, it would be nice to have an "import favorites" option to convert existing favorites to bookmarks. The editor/managment interface could use some work too. I'm sure everyone else will have a lot of suggestions, too