A couple years ago I picked up a mini when they first came out to play around with OS X. At the time it was OS X - Panther, pretty solid and have tinkered a little with Leopard, however upgrade annoyances & of course legality issues stopped it from being anything more than a weekend project.
Honestly though with playing around, if they ever sold Leopard for the retail PC market I would definitely consider switching my boxes (aside from my gaming rig at the time) over.
However factors to consider.
1) If they did sell it to the mass PC market and allowed OEM retailers like Dell to install OS X you will very quickly see OS X bloat up, similar to how Windows is. I'm not saying it will be as bad, but you have to remember, the OS is essentially built around a very precise set of hardware combinations. You introduce PC's and all its different hardware available you are bound to see some extra "girth" to it.
2) Apple is (and rightfully so) set to the initial computing business model where money is made from hardware.
3) Price for price with a typical enduser in mind, their prices aren't THAT far off from pc's.
4) They just set a record for their quarterly earnings. With a garbage economy, their sales continue to rise. Why would they tilt the scale so much by essentially reporting record earnings and then switch around their business model. Won't happen.
In summary, I'm really curious to how Apple is going to respond to this considering it is completely against EULA to do what they are doing.