The problem is simple: you start downloading the sdk from developer.apple.com and after a few minutes at around 500 KB/sec, the downloading speed drops and finally hangs.
Restarting the download with interrupt/restart works at ~500 KB/sec and then drops again.
Since it's a 2.5 Gb file, restarting every minute for an hour isn't exactly exciting...
Am I the only one with this problem ?