Those who follow BlackBerry launches closely probably have already guessed that the BlackBerry Javelin would be coming out on T-Mobile. It doesn’t really make sense for AT&T to release a 3G BlackBerry Bold only to launch a 2G BlackBerry Javelin months later. Additionally, T-Mobile tends to get Wi-Fi enabled BlackBerrys first because they fit in nicely with T-Mobile @Home.
It is, however, the recent API that all but definitively confirms that the BlackBerry Javelin is coming out on T-Mobile. The recent BlackBerry Javelin API includes T-Mobile WLAN error codes. I don’t imagine that RIM would code that elaborate of a bone to throw those speculating what carrier will get the Javelin off of the trail.