But, borrowing at 3% is little difficult to come by. Another way to look at it is that at today's retail rates for electricity, you get a 3% return on investment in a solar PV system. You'll get a higher effective rate of return if electric rates go up. This ties into inflation which might make borrowing even at a higher rate (than 3%) make sense but I'm not going to try to calculate that.
That said, venture capital has been moving into the solar PV market because solar is competitive at the retail level. One example where FedEx went solar is here http://www.powerlight.com/success/fedex.php.
At the corporate level that's fine. Big systems and big deals with risk management and all that. At the residential level, things are a lot slower. Enter a new player: http://www.citizenre.com/ which plans on renting solar PV systems to home owners for what their utility currently charges them for electricity. Their model for coming to market is like that of Amway: Multilevel marketing. They plan to begin installing in the Fall of this year (2007) and they are signing up customers now through a word-of-mouth campaign. How far will this go? I'm not sure, but they've doubled their customer base in a very short time (about a week) and they are approaching 3000 contracts now. That could be 3 million contracts in ten weeks if the viral marketing model works. I doubt that this can happen for practical reasons like server overload and the ability to build production facilities fast enough, but the eventual number of customers is not so unrealistic.
Another limit is the amount of net metering that states will allow as a percentage of total energy use. In Maryland, it looks like there is an out for the utilities at 34.7 MW of capacity http://www.energy.state.md.us/programs/renewable/solargrant/netmetering_statute.pdf which is not a lot.