I don't know of any workarounds or software upgrades, etc.
Perhaps check with the dealer, but I'm not very hopeful... Part of the issue is Nissan does not make their own head units. They are sourced through other mfg's like BOSE, Rockford Fosgate, etc. Nissan may not have access to or even the capability to change the programming in the radio and/or bluetooth.
Also check Nissan USA's website under the owners section:
http://www.nissanusa.com/bluetooth.html
And here:
If further assistance is needed, feel free to call Nissan Consumer Affairs at 1-800-647-7261, 8 a.m. - 5 p.m., Eastern/Central/Pacific Time, Monday through Friday, or e-mail us.
Please note that Bluetooth® support is limited to Nissan-recommended phones.
All versions of the iPhone including v4 appear to be supported.