I don't think that there is any need to wait.
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10 GigE is still pretty expensive compared to 1 GigE. The new Cisco Nexus 5000 switch is still running at about $900 a port. I don't think there is any danger in deploying now with a gigabit infrastructure. You should at least have 1 GigE, but there is no need to wait for 10 GigE.
Check out the entire iSCSI SAN FAQ.