Interesting that you should bring that game up, a friend of mine actually beat it, broke the bank on the Elemental Tower Defense game... we've been playing the flash versions online. I'm not saying that I wouldn't think about getting a PSP version for when I'm on the go, but the primary difficulty is that when you have figured out a dominant strategy (as my buddy did last night), you've got no further challenge coming from the game. They would need to spice it up a lot for a hand-held release that they could charge money for. Either that, or a super-cheap PSN download.
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To everyone else in the PSP forums, sorry this was kind of off-topic.
I wouldn't give a great recommendation for Field Runners. For the 7 dollars you pay for it, it's not really all that strategic. You just make the longest path possible using the towers and let it do the work. It's long enough for the money and it looks really good, but the gameplay's not all that strong.