I went to t-mobile store this past weekend to add a line. The sales guy ran through all of my phones and told me that Nexus one phone that I'm using right now is 2g sim card. From what he explained to me is that although my N1 is connected to 3g, the phone is constantly looking for 2g signals. Also he mentioned I would likely to see a speed boost. Indeed I saw a speed boost. I used to get around 780kbps but after this new sim card, I get 1.5mbps.

I bought the phone in June 2010 so if you bought the phone around that time frame contact t-mobile and see if you have 2g sim card.