5
I haven't tried this but this should work...
I believe you can change the category description datatype from XOBJ_DTYPE_TXTAREA to XOBJ_DTYPE_OTHER for 'cat_desc' in ./class/catHandler.php (around line 48) but in general it's REALLY DANGEROUS to allow html in input forms.
There is no way to enable html tags across the site independent of module settings. In my opinion it's just too dangerous to provide as a "blanket" operation.