How can I get boot details when booting from Snow Leopard DVD?
There should be a point—after you choose the language, but before it starts trying to install the OS—where you see a Utilities option in the menu bar. 



Do you get to this point? If so, what happens when you choose Disk Utility? 

There's more information about this at Using Disk Utility to Repair a Disk (University of Wisconsin).



OK, now it's a different question, so here's my different answer:

If repairing your main partition is what you want to accomplish, why not just run Disk Utility off the 10.5 system disk?