iMac 10.5.8 says its up to date, won't load 10.6

Here we go....

I connected my new iPhone 5 to my iMac, and it said I needed to update my software to 10.6. I checked, and I'm running 10.5.8. But when I run the update program it says there are no updates, my system is up to date.

No, actually there was an intermediate step....I tried to update iTunes first. That didn't work because of the 10.6 issue.

Anyone else having this problem?