Forwarding text messages?

Here is my situation right now, I am going to purchase an iPhone 4 with a brand new AT&T account. I have a T-Mobile account with an unlocked iPhone 3G.

I am going to forward all of the calls from the T-Mobile number to the new AT&T phone number. I also want to forward text messages, is this possible, or am I stuck carrying around two iPhones? I plan to keep the T-Mobile for one more month and use the number alongside my new AT&T number, during a "transition period" with my phone.

There are many reasons why I choose to do it this way, rather than porting the entire phone number.

A. There are some people who I just don't want to hear from, so a new number is perfect. (Ex. Political phone calls come election time!)

B. I need a month so people can reach me at the new phone, and it gives me time to inform them, and allow me to purchase the iPhone 4 on launch day.

C. I have business calls from clients that know me by my number, If anyone knows how much T-Mobile's ETF is, I may just go ahead and port my number over., depending on situations, if anyone can offer another solution to stopping political calls etc.