The only other thing I can think of with a boot loop is you didn't do the second part of the Simple Root. I had the same problem you did as well which I've posted a few times on here.
Step #1:
[Program] Updated: 8-5-10: 1 Click SimpleRoot OTA 1.47.651.1 Edition - xda-developers
Step 2:
How to start over: Fully rooted stock 1.47.651.1 in one shot (no adb!) - xda-developers
I had used Unrevoked 3 and then wanted to flash a custom rom, I couldn't flash any of them, not sure why. So I did the step#2 above and everything worked perfect from then on, no more boot loops or any of that. I think if you try the above it should work.
If you have already done step 1 and had no errors you could just try step 2, if that doesn't work just start over at square 1.
I should also say before doing step two I moved my NAND backup to my computer's hard drive, then formatted my SD card in the phone before proceeding with Step #2. A few people told me Unrevoked may have left something behind on my SD card, not sure how true it is but it worked so whatever.
I used Clockwork Recovery (Rom Manager) to flash Fresh successfully. I didn't do it from within the app though I made sure to flash Clockwork Recovery then chose "Reboot into Recovery" and then wiped Dalvik, then wiped data and then I chose the Fresh zip to flash. Hope this isn't too much info.