[eluser]Vince Stross[/eluser]
Can you post some code?
I'm not familiar with Google checkout and your cart setup but would it be possible to store your data as an array instead of an object? I always work with array's in the session but it's been awhile since I used an object. I know it's possible because I'm sure I've done it before, but I moved to arrays a while ago since they're supposedly faster.
If you can post some sample of your code that would be really helpful, otherwise - no, nothing comes to mind.