I asked the same question. If you are with Nextel here is your answer. This is from AvantGo support:
Make sure that your device is enabled for HTML web browsing (some carriers offer only WML/WAP browsing). If your carrier does not support HTML web browsing, HTML browsing may be turned on through your corporate BlackBerry server (BES) when the Mobile Data Service (MDS) feature is turned on. Contact your company's IT department to inquire.
The following is a status list(as of 4/15/05) for North American carriers. Carriers marked "yes" support HTML web browsing without the Blackberry device being paired to a corporate BlackBerry server with the MDS feature turned on.
Drakeford, have you found AvantGo for BB to be at all useful? I liked the Palm version, but found the BB version to be of little practical value. Of course, it's more than possible that I might be missing something.