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Since this question was posted, the iPhone OS 3.0 beta has been released on the world of developers - and the situation here is different. If you put the beta OS on your iPhone you cannot remove it. This is not true of the iPod touch, although caution is advised there too. OS 3.0 beta, at time of writing (now on beta 3), is steadily improving, but still has many issues, so you really don't want to be relying on it as your everyday phone!
Once 3.0 goes live, you will be able to move to that, of course - but you can't go back to 2.x due to baseband differences.