There has been a fair amount of discussion about WHO he is, "Is he House's biological father?" I have doubts.
Here's why....

This guy looks to young to be Houses father. We met John House in Season 2 and he was a MUCH older man. Also MOTHER House is older than this guy.( Unless he's aging backwards )
House knowns what the man he suspects of being his biological father looks like, as he was 1) A friend of the family 2) he was at John Houses' funeral. Where House pointed him out to Wilson.
AND if for some unknown reason House forgets what he looks like, the pic on the dust jacket of the "book of sermons" would have reminded him. It did Wilson.
The patient says "You're Dr House" which indicates he has never seen House but has heard of him. And House's reply of "Don't tell me we've dated" suggests he does not know the patient either. If he is Houses father then surley he would recognise him from the funeral. Now for the kicker : This guy looks like he is about the same age as House, could he perhaps be related ? Maybe a half-brother !!!! If he is, it could mean the writers are going to introduce us to Houses biological dad sooner rather than later. OR is he maybe Lydia's hubby. WE know he travels alot, and she might have mentioned House, (not the chair sex though). If she is the one House has a 1 night stand with at the end of the season. Why would she suddenly re-appear ? She lived in Philadelpia and moved to somewhere in Arizona. What is she doing at PPTH ? Was she just in the neighbourhood and decided to pop in for a coffee and a shag or is there another reason? Maybe I am over thinking this, but it is definatley something to think about.