"GSM auto (PRL)" works on AT&T Nexus, I don't know if it improves the battery life over "WCDMA preferred"
Looks like this process works by improving battery life when phone is idle. Here are the instructions from the first original post.
1) Goto dialer and dial *#*#4636#*#* (DO NOT press send)
wait a second and you will get the test menu.
2) In the test menu choose 'Phone Information'
3) scroll down and choose on the first drop down - GSM auto (PRL) (for high speed data) or GSM only (You may get 3g if not use GSM auto (PRL)) Don't know what PRL means, but GSM auto (PRL) allows High speed data HSDPA (it was previously on GSM/CDMA auto(PRL) I think.)
4) Back all the way out.
5) You might want to reboot, but I found the settings worked straight away.
Edit 3 - I don't take any responsibility for step 6 -
6) Try other settings on the dropdown if you are brave as I don't know the consequencies of using settings like "EvDo only" and "unknown" as they may be better depending on the way your provider provides information.
Edit 4 - can't seem to select the other settings like EvDo and unkown anyway.
Edit 3 - TBH - GSM auto (PRL) is working for me with HSDPA and I will try (PRL) instead of GSM auto (PRL) <- edit - previously it mistakenly said GSM only the CDMA and WCDMA settings later to see what other effects there are.
Your results will vary depending on the rom, but it should improve battery performance - some have had better results than me using this.
GSM only <---- If you don't get 3g with this it is the same as 2g setting then use GSM auto (PRL) if you want faster than 2g (as some people are getting 3g on GSM only - I will investigate. (if you don't use internet or don't need fast internet)
5½ Hrs - lost 5% Battery - This one is same as running 2g as you don't get HSDPA, but using Opera browser makes browsing fast even using this. If you want Higher speed data use the one below.
GSM auto (PRL) <---- Recommended if you want High speed Internet and save battery
1:40am to 2:00pm (12hrs 20 mins) - lost 40% battery with moderate use a bit of surfing at HSDPA speeds, a few calls and messing with a few apps. Other people have had even better results than me though, so I might change my rom and try again. (I would usually about this amount of time I would have lost about 70-75% - and yes it was that ridiculous - I had to find something to stop this).
" WCDMA Only"
2:06pm to 11:37pm - (9hrs 31mins) lost 46% battery again moderate use and a reboot
This one might be useful if you live in a place that has low GSM signal like the chap in post #10 as it does give HSDPA - but this is only a suggestion as I cannot test it in Iceland. Anyway
" WCDMA preferred"
6:02am to 2:51pm - (8hr 49mins) lost 41% battery this time with less use that the WCDMA use and no reboots.