Hi Esben,
First, Mono 3.0.3 is still a beta release. My recommendation is that you should stick to 2.10.x.
Second, as far as I know, Mono 2.6 -> 2.10 -> 3.0 each has significant changes in TCP/IP layer. So if your environment can be used as a test bed, please carefully see whether this issue is reproducible on all typical Mono releases or it is 3.0 specific. If it is 3.0 specific, we will have to draw attention from Mono guys.
The GetResponse method hasn't been changed for a long time.
Regards,
Lex