I think this is a fabulous idea. Don't get me started on the idea that throwing more money at the problem isn't going to work. Let's get the money and then work at making how and by who the money is spent better. I know that's a gross simplification of the problem of "aid". I have read books/papers on this subject (Thanks a lot Bono!) so I know a little bit about all the major arguments for and against foreign aid.
If the banks fight a 0.05% tax on financial transactions they'll just be proving that they don't care about anything but themselves (some people still need to be convinced). Check out the website and sign the petition to tell politicians you support the Robin Hood Tax.