Wirelessly posted (BlackBerry 8700c: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0) BlackBerry8700/4.1.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102)

Select the 'System' font and save it. Then pick whatever font you want and it will be smaller than the listed smallest size.

You can tell that it's smaller by looking at the settings screen. With BBCasual on the normal size 7, there are 13 options listed on the screen. After doing the 'trick' there are 15 options listed on the settings screen.