FindLaw has an interesting post with 5 things to know. Judge Jones refused to stay the order pending appeal, so SSMs start today. Also, Pennsylvania must now recognize out-of-state SSMs.
How does this impact employers? Most notably, same sex spouses will now "count" for FMLA purposes (e.g. employees can take leave to care for a same sex spouse with a serious health issue). Also, employer benefits and retirement plans likely refer to "spouses," which will now include same sex couples. There may be some tax implications too, but that's not really my thing.
I will note that this decision can still be appealed, and that another court may issue a stay pending an appeal - so this is not necessarily a done deal yet. Also, it's still important for Pennsylvania employers to keep up with other state's SSM laws for reasons listed here.
UPDATE: Eric Meyer notes that same-sex spouses will now be covered by PA's mini-COBRA. Nice catch.