Update (6/27/2013): What about Pennsylvania employees who live out-of-state where they are legally married to a same-sex spouse?
Hollingsworth v. Perry - The Prop 8 Case
Hollingsworth's impact on Pennsylvania employers: The important thing to understand about this decision is that the Supreme Court did not address a constitutional right to same-sex marriage or the constitutionality of a state law banning it. The majority based its decision solely on standing. Accordingly, unless you're an employer hoping to intervene in a case to defend the constitutionality of a statute (in which case you probably lack standing), this decision will not impact employers.
(Note: This may impact California employers pending new legal wrangling over Prop 8).
United States v. Windsor - The DOMA Case
Now, let's look at United States v. Windsor (opinion). The Supreme Court held that a provision in the Defense of Marriage Act (DOMA) was unconstitutional. Specifically, the Court struck down s3 providing that "marriage" and "spouse" under federal statutes meant only heterosexual couples. The SCOTUS majority held that the law violated the equal protection and due process rights of same-sex married couples, with an emphasis on states' rights to define marriages for themselves.
Windsor's impact on Pennsylvania employers: It's important to understand what Windsor did not do. First, the holding was expressly limited to those "who enter into same-sex marriages made lawful by the unquestioned authority of the States." Also, despite headlines that SCOTUS struck down DOMA - it expressly left untouched s2:
Section 2, which has not been challenged here, allows States to refuse to recognize same-sex marriages performed under the laws of other States.So, we have a pretty limited holding. Generally, Congress cannot deny federal benefits to same-sex marriages made lawful by a state, but states are still not required to recognize same sex marriages from other states.
Let's focus on Pennsylvania now. To my knowledge, Pennsylvania does not recognize any same-sex marriages. So there exist no "same-sex marriages made lawful" by Pennsylvania. And, Pennsylvania still has no obligation to recognize same-sex marriages from any other states. If anything, this decision supports the proposition that defining marriage is within the authority of the state.
So, I can't think of how this impacts Pennsylvania employers at all. I'm open to feedback and criticism, so drop a comment if you feel differently.
(Note: This decision may impact employers in states that do recognize same-sex marriage. For example, federal laws addressing benefits (ERISA) and family leave (FMLA) will likely be interpreted as defining "spouses" to include those lawful same-sex marriages).