Unfortunately I can see that problems with downloading from sourceforge.net today and the reason is Sourceforge.net service. They were hacked and due some reason from the attack recovering, all new uploads on Thursday/Friday/weekend have these issues as far as I know :-(.
Look at http://sourceforge.net/blog/sourceforge-attack-full-report/. They don't have any exact ETA for fixing the issue - they said to me that sometime this week.
Try to download Seam with wget or any other download manager, which can resuming download.
Or in the meantime I have uploaded the release at download.jboss.org:
Sorry for that!