I see plenty of threads about the signature not working in IE7, but it appears that the general consensus is that they work in IE7.
They do not work for me nor anyone that I have talked to. They are still broken. Is this actually being addressed or have the developers just said that it works for them and they don't want to look at it any more? I see plenty of tickets in the ticket system where the developers have just closed the ticket saying it works, when it clearly doesn't.
Have you installed recent SVN release? Signatures work for me in IE7.