Hi,
I think that you need to get your terminology correct. A
logical partitions is the same thing as a
Primary partition. See
Disk partitioning for more info on that.
Everything looks good to me. I just don't know how the well the boot-loaders are going to play with each other. I know that you won't have a problem with GRUB. But Longhorn doesn't always play nicely. It will be interesting to see.
Cheers!