I have Visio 2003. If I do a search for "text box" in the "Search For
Shapes" window (right above the stencil window), it comes up with two
different options.
1. Text Box (multi-line) and
2. Text Box (single-line)
Found exactly what I was looking for in this:
Click on Text Tool on the tool bar (it is represented by capital "A"). It
is normally located next to the Connector Tool on the tool bar.
The text box (multi-line) and (single-line) as you suggested is also great
but I think they're meant for designing GUI.