Yes..you can restore (or upgrade) to the legit apple firmware and
jailbreak..subseq.unlock and activate using pwn-age and a custom ispw file...
So you'll need a good original 2.0.2 FW file in any case...
When you restore in iTunes the apple FW file is downloaded somewhere in your apple install folder...or in your 'documents and settings' folder...
Here's a direct apple link to the iPhone1,1_2.0.1_5B108_Restore.ipsw
file... in case you're not able to find it...
http://appldnld.apple.com.edgesuite....8_Restore.ipsw
Hope this helps...
Cheers...
Wallyİ...
Edit :
CAUTION !!!!
There is a new 2.1.0 firmware upgrade now...
I don't know whether you can downgrade to a custom 2.0.1 firmware AFTER restoring to 2.1.0
But I could be wrong...