Well... duh. Sony isn't doing this to be "evil" or to box-out would-be homebrew game developers. It's for a much, much simpler reason: copy protection. If it were possible to run anything you wanted on a PSP people could write little hacks that circumvented the code signing. Code signing presents a chicken-and-egg problem to hackers that is a solid first level of defense against game piracy. Since game royalties are probably expected to make up a huge portion of Sony's profit on the PSP it makes sense for them to do everything possible to prevent people from stealing games.
Why am I so passionate about this subject? About a year ago I released a game for Pocket PC, SmartPhone and Symbian called Vector Blaster. About a month after I released the game I started surveying the p2p networks and noticed that trading of my game was rampant. I estimate that for every one person that bought Vector Blaster there's at least one hundred other people that stole the game. Myself and one other developer that I know basically stopped making games for Pocket PC and SmartPhone because piracy was such a huge problem and Microsoft ignored our cries to help lock the platform.
Vector Blaster isn't even that expensive of a game. On PSP, the drive to pirate is even greater. The games cost more, they're higher quality, etc., etc.--there's just more at stake. Sony needs to protect their platform or no one will make games for it.
The general rule in the game industry is 90% of a game's profits come in the first 6 weeks of sales. (MMORPGs being an exception). PC game companies shoot for the 6 week mark for their copy protection. If they can delay the piracy of their game for 6 weeks then it won't make a substantial impact on their sales. Sony is probably trying for a similar goal. I bet that if they can delay the cracking of their platform for 12 months then that will give 90% of their target market a chance to buy a PSP and a few games. If it's cracked after 12 months it will definitely have an impact on sales, but the impact on Sony's bottom-line won't be as significant.
In the end, someone will have figured out how to get past the PSP's copy protection efforts, but by then Sony will have sold enough games to recoup the cost of the PSP and bring home some profits. Or not. Sony could get lucky: If it requires a modchip to crack the PSP then I doubt game piracy will have any effect at all on game sales. I just can't see kids asking their parents, "Mom, can I take my PSP apart? ... But Mommmmmm...."