After the 1.6 updates I haven't been able to join any of the servers that have updated to the 1.6 updates. I know a few other people are having this problem as well. I have tried updating my java and redownloading the .minecraft. I know it isn't a firewall problem as I have allowed java and minecraft to be usable on my computer. All version below 1.6 still work and I can still join servers on 1.5.2.