Without that plan--you can't use the apgs feature in the phones. There's no way to access the gps capabilities. As opposed to phones (mainly not available in the US except from Sprint/Nextel) which actually have on-board GPS that can be used stand-alone...
That is generally false. I (and many others I know) have never once paid for the cell companies GPS service on any cell phone, and I frequently use the GPS features on my current, and previous phones.
What is correct is that AT&T (and probably other carriers as well) does try to block the GPS unit on some units within the phone software so that it will only work with their telenav software (BlackJack II with windows mobile 6 was locked) and sometimes require a hack to access the GPS unit from programs like google maps or Live, (the windows mobile 6.1 software update, on the same exact phone, required no hack and the gps would work with any program)
From Wikipedia Assisted GPS article:
A typical A-GPS-enabled cell phone will use a data connection (internet, or other) to contact the assistance server or a standard network connection for A-GPS information. If it also has functioning autonomous or standalone GPS, it may use standard GPS, which is sometimes slower on Time To First Fix, but does not lead to network dependent downsides, such as failure to work outside of network range, or charges for data traffic. Some A-GPS solutions do not have the option of falling back to standalone or autonomous GPS.
You are correct that without a data connection to the cellular network (it uses the cell tower to determine it's starting position) the "assist" feature will not function on the A-GPS receiver, but virtually all gps unites in phones are of the type that features a standalone receiver as well, and while they may take a long time (I've personally seen 30+ minutes, and heard of 45+ minutes in extreme cases with weak signals) to get the initial lock without the assist, they will still get the lock, and once done, they will operate as any other GPS unit.