[mojen]

I was searching online for a solution to this problem and found this thread. Here's the method I used in case it might be useful to anyone else.

1. Set up a free account on Toodledo.

2. Use Toodledo's RTM import tool to free your tasks from RTM: http://www.toodledo.com/info/help_rtm.php

3. Export a text file of your tasks from Toodledo.

4. Import the text file into OmniOutliner.

5. Save OmniOutliner file.

6. Import OmniOutliner file into Omnifocus. You'll need to map the column headings.

Job done. Farewell, RTM. And thanks, Toodledo. :)

[edited to add OmniOutliner steps]