The main advantage to getting it unlocked is that you maximize your chances that you will receive a phone with its full feature set intact. Looking at how T-mobile USA handles its subsidized Android phones, T-mobile has a say in whether a Google firmware update gets pushed to T-mo branded phones, and what is included in those updates. They have put pressure on Google to remove tethering apps from the North American Google Android app store. T-mobile tends to lock down other things on their subsidized phones such as bluetooth tethering and the ability to download non-Tmo approved apps. I'm not sure if T-mobile will take kindly to free VOIP.

Looking at the chatter on other T-mo sites, it sounds like the release of the subsidized n900 will be delayed until Q1 2010, in part to get it compatible with "Hotspot at Home". If true, that tells me that there will be some firmware customization being done for T-mobile, which is exactly what I am hoping to avoid altogether. Hopefully, for those waiting for the subsidized device, they won't do any additional hardware customization, like removing the front-facing camera, lowering memory, etc.

With all that being said, I don't want to sound too alarmist: as others mentioned, it will probably be very difficult to lock down a phone running on an essentially open platform. And obviously, the hacker community is quite strong here: any workarounds to most feature-locking will likely be circumvented in short order. So, really, it could be just a case of if you want it now, then you'll pay more. If you can wait six months or so, you'll pay (possibly significantly) less (though if you buy subsidized, you may have to hack your device a little to get it to perform as intended.) As has been mentioned in other threads, the other advantage of waiting is that you'll probably get a more stable device with a higher build quality, as Nokia irons out the kinks by using the early-adopters as de facto beta testers.