Not sure how teh other networks work on porting, but when I moved from Orange 18 months ago, my N96 took 3 weeks to show the correct number under the SIM info.
About the same time I had issues with my Tmobile logging in, as the temp number no longer existed and so my account info for my Tmobile went screwy.
I guess the change only happens when they re-issue the number, thus completing the porting process.