Got this one from the Microsoft newsgroups:
If you make a shape and add centered text to it, the text will stop being centered when you reach a Width of 52 inches (~132cm).
This seems to be a problem with Visio 2003 and Visio 2007.
The 52-inch limit is a "printed size" limit- ie: if you have a drawing scale of 1:4, then the problem will occur when the shape is 52 x 4 (=208 inches)
Right-aligned text also seems to have this problem.
My machine:
Visio 2007 Pro SP1 (en)
Windows XP SP3 (en)