We are currently using (CentOS5_2V4_1_10)ami-cb52b6a2, but I am hoping to get something more updated.
I think that is the latest Amazon-provided version; they are well behind on supporting more recent versions due to issues with making the EC2 Xen virtualization work properly with newer kernels.
You might find that another vendor has provided a public AMI with a more recent version if you look around.
My company uses Fedora, and Amazon's latest AMI is only Fedora 8 even though Fedora 11 is now available. I got around this by updating all the packages to Fedora 11 but leaving the kernel at Fedora 8; you might be able to do something analgous.