In the dog days of summer last year, when balky downloads in cities across the U.S. were slowing iPhones to a crawl,’s Gadget Lab launched a survey of 3G network speeds to try to pinpoint the problem.

Gadget Lab invited iPhone owners all over the world to run a simple network test and submit their findings electronically.

After collating the results from 2,636 users — 62% of them in the U.S. — Wired concluded that the fault lay not with Apple’s AAPL device, but with the U.S. carrier it had chosen to partner with, AT&T T . See the results here.

Now, following rumors that Apple has been talking with Verizon VZ , the Gadget Lab has decided to give it another try.

On Monday, it launched a second Battle of the Carriers, this time testing all the major U.S. carriers — AT&T, Verizon, Sprint S and T-Mobile DT — and other 3G smartphones, including Research in Motion RIMM BlackBerry.

The exercise only takes a few minutes. The instructions, available here, are fairly straightforward: you go to a Wired website on your phone, take the test, and then record your results on an interactive Zeemap. Just remember to click the “details” tab before you submit your entry, or else you end up — as I did the first time — sticking a blank entry on the map.

Below the fold: The 2008 results in map form.