vim: is there a plugin to show all of your TODO tags in source?

When you say grep, are you talking about grep from the command line, or :grep in vim? The latter allows you to view a quickfix list of matches and jump to them. If typing out :grep TODO -r . becomes cumbersome you could easily map that operation to a hotkey.

I've not tried it (yet), but this plugin looks promising.

Edit: I've just tried this plugin out. I will be keeping it around. It's very handy and interactively moves you around the buffer while you navigate the task list.

Also, there is a mirror on github if you use pathogen or another means of git submodules for your vim directory.