iTunes used to keep multiple backups for iPhones, but now it only keeps the most recent (plus whatever ancient backups you had before iTunes changed to keeping only 1). You are probably out of luck unless:
a) you haven't backed up your phone at all since the restore (not sure if it happens automatically or not)
b) you backed up your computer's hard drive recently. If you have a recent backup of your computer, you can go in and extract the most recent backup from that (assuming you backed up your iPhone once in a while)
It's generally a good idea to manually copy your iPhone backup to a new file name (I usually add the date to it) on occasion, so that you can recover from these types of mistakes. Keeping only 1 backup is very dangerous - especially if you have automatic backups turned on!
One time I was trying to repair an iPhone, and ended up restoring it as new on another computer. After it was finally up and running, I figured I would just hook it up to my regular computer and do a restore from backup. Unfortunately, when I plugged it in, it immediately did an automatic backup of the completely empty iPhone, and overwrote the very backup I wanted to use! That's when I learned my lesson... Whenever you are doing funky stuff to an iPhone, make sure you manually copy your critical backup to another location other than the default before you do anything.
