Please confirm this for me and then I will send it in.
Start a new drawing in SW SP3.0 (2.1 & 2005 Beta both work ok.) Put in a note, dimension, weld - anything that uses a symbol library, like centerline, fillet weld, etc.
As long as the first library you pick is Modifying Symbols, you can then pick another library. But then after that, the box in which to pick another library is grayed out. Also, if you pick a library other than Modifying Symbols as your first, you then have no other choices. It doesn't seem to matter what you use the symbol for, just the fact that you don't have the opportunity to pick a symbol library after using one other than Modifying Symbols.
WT