I've released Thimblr — a tool to speed up your Tumblr theme development. Its a Ruby gem, so it's very simple to install, with gem install Thimblr, once that's done you just need to run thimblr, as a binary is installed for you with the gem.
This screencast should help you with the basics, though I've designed it to be totally self explanatory, but if you have any problems please open an issue on github and I'll get right on it!
Please note, because I don't have a windows box to test on at the moment it just plain won't work at the moment. If anyone knows of a good place to store settings files etc. in windows then let me know and I'll get it working.