I had a look at the other posts regarding the use of RTM with a Blackberry and didn't see the following mentioned, if it has already been covered, apologies, but it may be useful to some.
To get your Blackberry to alert you at the time a task is due:
1. Set RTM to send email reminders to your Blackberry address at the time a task is due.
2. Create a new profile on your Blackberry that uses a loud alert for incoming messages.
3. Create an exception on your Blackberry for any messages that come from the address email@example.com to use the new profile.
Now whenever a reminder email from RTM gets delivered to your Blackberry, the Blackberry will beep loudly alerting you to your task.
Hope this is of some help,
N.B. This also works for Google Calendar.
Posted at 9:22am on October 11, 2007