I would imagine this type of thing is going to happen more often among YC startups as the number of them grows. Both straight mergers a la Infogami/Reddit and, probably even more likely, founders from failing/faltering YC companies jumping ship to join other YC startups.
Didn't the same thing happen to YC company Auctomatic?
I'd love to see more mergers between IT and non-IT startups. Obviously most wouldn't be a good fit, but the ones that are could be very interesting.
What if there is a cultural mismatch between the two companies?