Oh i like that. one person has put a link on here to another thread
and the end solutiion is a link back to this thread. Anyway! there is a
cheeky solution to this problem in infopath. Its not a rich text box
but it is the next best thing if you want a multiline input.
Step 1: create a standard text box for the field in question
Step
2: click the base of the text box in question and drag it so its height
increases to the desired sized and it looks like a rich text box.
Step 3: double click on the text box in question so it opens the properties box
Step 4: goto the tab marked "display"
Step 5: You should see tick box options for "Multi-line", "Paragraph Breaks" & "Wrap Text". Click all three.
You should now see that it acts very much like a rich text box as an input.