There is a tip in Brian's book on using HTML to format part of a text field.
To add a carriage return, use ChrW(13). 13 is the ASCII code for a carriage return.
So, your formula would look like:
"<b>This is my bolded text</b>" & ChrW(13) & "This text is plain."
You then need to go into the Format Field dialog. Go to the Paragraph tab. Under Text Interpretation, make sure "HTML Text" is selected. Otherwise, it won't interpret the HTML tags.