I fail at waiting! I just ordered the LE version this afternoon! Hopefully, I'll see it around Monday or so.
There's no major differences that I know of between the US and Japanese PSPs, at least for the older version. The slim version may have more, but nothing major.
You'll still be able to play US games on the Japanese machine, you just won't be able to play non-Japanese video UMDs.
The one difference that's immediately noticeable is that the X and O buttons are reversed for the PSP's menu and Japanese games. American games will still use the X and O buttons as normal.