Maybe it's from interest that I just noticed that pings to the n900 vary extremly depending if I initiate a sip or a skype call.

With PSM enabled:
Pings to the n900 normaly take ~ 140ms

If I start a sip call pings take between 40ms and 140ms an go up to > 9000ms (if they don't start between 2000ms and 9000ms right after the call is established)

If I start a skype call pings take betwen 3ms and 5ms and sometimes go up to 4000ms.