I was struggling with this yesterday, and happened upon this link, which explains how to make it do what you want (at least, it worked for me :) )
The technique described at that link is simple:
- Add a "scrolling credits" title thingy to your video
- Select all the text in in and copy to the clipboard (use cmd-a, cmd-c, for example)
- Open a new RTF document in the "TextEdit" application
- Paste the selected text into the new document
- Open the "Page Setup..." dialog from the "File" menu
- Select "Manage Custom Sizes..." from the "Paper Sizes" drop-down
- Make the paper size 24" wide by 11" high
- Edit the formatting and content of the document to whatever you want it to look like in your movie
- When done with that, select everything in the document, and copy to the clipboard
- Switch back to iMovie, select the "scrolling credits" clip you added earlier, and select all the text in the clip again.
- Paste the text from your clipboard over the selected credits text
- Except to adjust the length of the credits clip, DON'T TOUCH IT ANY MORE, or iMovie will (probably) mess up the formatting!
If you need to make changes to the text or formatting, go back to the TextEdit, document and edit that, then copy-paste back over the text in the credits clip in iMovie again. Lather, rinse, repeat until you get what you want.
Upon typing it all out, I realize that some of these steps may be superfluous but IWFM, IANA iMovie Expert, YMMV, ROFLWTFBBQ and all that. You probably don't have to bother with copying the original credits text into the RTF document - you can probably get away with simply typing them into a new empty doc, formatting, then copy-pasting over the original text in iMovie. Also, I don't know if changing the page size is necessary, or if the 24"x11" measurement has any particular significance...
Feel free to experiment and suggest edits or improvements to this answer. :)
@grgarside - thanks for the reminder to fill out the answer!