Are you sure you don't have Tiger? Mine came with Tiger (10.4.8) install disks -- same as the version pre-installed on the hard drive. Since our iMacs were built the same week (and 2 months before Leopard released), I'd expect them to have the same install disk versions (+/- 1).
I bought mine before L-day, so I had to order the "free for $10" UpToDate DVD from Apple. I'm guessing smalldog sent you the same update disk (rather than the full retail version of Leopard). Is that correct?
...enjoy the new toy,
LK