There is good news and bad news and good news.
The good news is even though your motherboard has been out for 2 years and is AM2+, Asus provided a BIOS update so that you may install a Thuban.
The bad news is that the BIOS update, while recognizing the 1090T, is in need of some serious tweaking.
The good news is a beta BIOS for your motherboard is currently being tested by both Asus and NV.
Beta BIOS Releases in Validation Testing as of 7/30/2010-
M3N-HT Deluxe - Improved Thuban Performance, NV involved, still in beta testing as of 7/28
From XtremeSystems: ASUS AMD Beta BIOS Releases
Without going into tremendous detail, this is not a simple task. AMD Turbo on the Thuban microprocessor adjusts the CPU multiplier and voltages to each core. When you increase the base multiplier on the 1090T, you increase the Turbo multiplier, too.
Hopefully, the beta BIOS in testing will incorporate the changes needed for you to maximize the potential of your system.
edit: I forgot to add that "bingo13" in the link above works for Asus support, and for the past three months has been working very hard on Thuban BIOS updates.