I bet the best way to try this is to have the 1.0.2 install file downloaded to your computer.
1.0.1 install file:
http://appldnld.apple.com.edgesuite...70731.Nt6Y7/iPhone1,1_1.0.1_1C25_Restore.ipsw
1.0.2 install file:
http://appldnld.apple.com.edgesuite...70821.vormd/iPhone1,1_1.0.2_1C28_Restore.ipsw
Let it download the 1.1.1 update (click the "Download Only" button), but unplug your phone while it's downloading. Once done downloading. Disconnect from the internet.
Rename the 1.0.2 install file to the file name of the 1.1.1 file it just downloaded and put it in the same directory as the 1.1.1 file replacing it. (if you can't replace it, close iTunes first)
Plug your phone in and let iTunes Restore the phone...it'll install the the 1.0.2 file thinking it's the 1.1.1.
I wouldn't reccomend doing this on a phone from 1.1.1 to 1.0.2 however.
-Derek-