I have ran into this question myself and thought I might share the tip that was given to me from AutoCAD developer Kurt Chase. When you have Mtext you can use the Enter key to break the paragraph or add a space. But when the Mtext is in a AutoCAD 2005 Table Enter moves you to the next cell. What you do is just like in Microsoft Excel and use ALT+Enter.
Link to my blog post on using the AutoCAD Tables and Mtext to achieve multiple columns of text. Link Here