Isn't it plain irritating when you thought you finally figured out what john mayer did out there, then he shows up and do something that you might never figure out ?
Though I agree that tone is more in the fingers than gear, one still can get wee bit closer to the character of his sound by using some of his gear.
Gear-wise IMHO, I think that an alder-bodied, rosewood fretboard strat would do well. JM also has that big fat strat tone with subdued mids. The overwound single coils like the texas specials will work, but connected to some kind of eq pedal to tame the mids to get the scooped sound or something, and I also noticed that he sounded like he turned down his tone knobs on his guitar quite a bit. Upsizing to string gauge 10s instead of 9s will do alot to help.
For the amp, nothing sings like a tube amp, the fender blues junior will be a good option, and there are many mods you can try out on it to get what you like, its all in the web.
JM use so many kinds of pedals and drives but one main stay in his set-up would be the keeley ts-808. I made a clone of it myself and I personally find that it sounds twice as good than the original in an A/B comparison. Again, all the clone info are readily available on the net. I like the keeley BD-2 through the 808. There is some compression & reverb in his sound too.
Playing-wise, an arsenal of Hendrix's style and SRV licks will help a ton. But what I think contribute to JM's tone come from this fingers, he tend to switch to fingers alot as seen on the Crossroads City Love intro,
http://youtube.com/watch?v=IEE2_RYyVYk. I suspect he uses two fingers (index & middle) together when he plays the higher strings, or his thumb for the bass strings. And sounds more like 'popping' as opposed to finger plucking the strings. His technique is somewhat similar to Warren Haynes playing
http://youtube.com/watch?v=iChAWaX5ZDo. That fat fat sound.
For his electric playing, apart from learning blues, learning funk and R&B will help you understand what he does. But for his acoustic playing, thats a different story....... its totally another world of JM style !
He plays jazz as well........ you can hear those jazz licks on the 200 guitars video on youtube.......... He once mentioned on his web that if everybody were to do something uniquely their own, todays music will be so much more interesting.
If you have big long fingers, you are already halfway to that sound.... think SRV, HENDRIX, MAYER.