The final OFFICIAL Asus Firmware for the device is 4.0.3, released in July of 2012. The build number ends in .27
There are LOTS of custom ROMS that go beyond 4.0.3. I have been running TeamEOS 4 since about December. It is Android 4.2.2 at this point, and is very stable despite being a "nightly" build.
I have this thread that details how to install it as you will need to root and install a custom recovery.
There are other custom ROMs you can find at XDA and other sites. One of our staff members has a hybrid ROM that has CyanogenMod, ParanoidAndroid and AOKP features on one ROM. It is still a bit early for it, but he is working hard to get it fully functional. Curretnly the biggest issue is the keyboard dock function keys are not working, but beyond that it is fully functional.