Just noticed yesterday that my VOIP handset was no longer registering with Sipgate. It's a Siemens N510 Pro and was showing "registration failed" in the web UI for the basestation. This behaviour has only manifested since recently updating my WRT32X to build r12235. Previously i had been on r10307 since June last year, so a big jump, I know...
I did a little tcpdump'ing (VOIP telephony is my day job, lucky me!) and saw REGISTER requests going out but nothing coming back in. Previously I had no port forwards for this device configured and everything worked fine. I set up SIP and RTP port forwards to my basestation as per the Sipgate FAQs and everything is working fine again now.
So, no help required or problem, just posting this in case anyone else runs into a similar issue as something has obviously changed.
EDIT Spoiler: it was Software Flow Offloading. I've now disabled it and the VOIP service is registering without and SIP port forwards.