That's right. The only way to get AIM on a BlackBerry without a BlackBerry plan, is via the SMS text messaging system or WAP. However, Nextel BlackBerries doesn't have outgoing SMS text messaging, so you're stuck with WAP. Which is rather inconvenient.
If you want a luxurious AIM experience, with the ability to run "always-on" nonstop 24/7 in the background, and without needing to refresh a WAP page, you should consider a BlackBerry email plan, which will give you access to superior chatting client such as Verichat: BlackBerry Instant Messaging FAQ
(the best AIM for BlackBerry!)
And who knows, you may find the email much more useful than you thought. You may want to use your BlackBerry email as a separate "emergency email" address. BlackBerry is famous for having pager-style email without needing to check email - the email is automatically pushed to the BlackBerry, which means emails can show up on your BlackBerry in just 5 seconds after someone clicks "SEND" on their desktop computer. If you are afraid of spam, you can just make sure you only use a separate address and only give it to your friends and coworkers. You'll get addicted to the BlackBerry convenience relatively quickly. Even if you only do 4 or 5 emails per day, you'll wonder what you did without it! BlackBerry has no real email size limit -- Many people can even email eBooks/shopping lists/bus route timetables/etc to their BlackBerry and read from the BlackBerry when they step out! You'll think up uses that you never thought of!