http://www.grateful-dead-picture.com/images/Garcia-Elvin-Bishop-sm.jpg shows him with Jerry Garcia in 1978.
Google turns up this from circa 1958:
Though it’s not clear from the picture, the advert says that the figure is wearing a backwards baseball cap.
I was careful to use inverted commas on that occasion 
samclem, I was unable to open that .jpg. Was he wearing a ball cap? I was thinking more along the Butterfield Blues Band era–Mary, Mary, you know.
But on a baseball catcher!
My husband often wears his cap backwards when riding his motorcycle without a helmet – this is to keep the wind from catching the bill, of course. I’ve seen old pictures of other bikers, from as long ago as the '30s (judging from the bikes). So I’d have to call bullshit on your friend, vinniepaz.
However, while looking for a picture of a biker in a backwards cap, I ran across this essay. It’s a spoof, of course, but since your buddy doesn’t seem too terribly bright, maybe you can pass it off as truth and collect the cash prize.
I’ve got a photo of my grandfather driving off to a Stanford-Cal football game with his"newspaper boy" cap on backwards in 1911.
I would imagine that wearing one’s billed cap backward originated approximately 5 seconds after the inventor encountered a breeze, like Spartydog said.
ok ok ok im sorry i should have been more specific but i didnt know everyone here was so literal…
the first person to wear a hat backwards as a style statement
Style statements? Did they have those back in the seventies?
Well, my offering of Matthau wearing his ballcap backward all the time in The Odd Couple in 1965, even though he was not riding a motorcycle and afraid of the wind, nor was he a baseball catcher, might qualify. Due to the popularity of the play, movie and tv show, it could have caused a trend. If Norm Jewison wore his backward on his sets, before 1965, then I’ll slink away.
RM Mentock. Elvin was wearing a stupid hat in that picture. Not a ball cap.
Just go to Google, type in “Elvin Bishop” and hit images instead of ‘search web’.
This is irrelevant, but in his novel Pattern Recognition, William Gibson has a humorous description of a professional “coolhunter” who is so good she tracked down “the very Mexican who first wore his baseball cap backward”. It sounds cool, until you think about it and realize there’s really no way to prove who did it first.
I think the character’s name from the “Little Rascals” was Scotty. I think.
Easy, the first person to use a hat backwards was the first person that used a hat; I guess Og the caveman was not too picky about fashion standards and wouldn´t really mind how the new advance in technology sat on his head. 
Who’s to say that the bill belongs in the front?
But, aside from that, my guess would be Douglas Allison, catcher for the 1869 Cincinnati Red Stockings.
Probably many before him–the Red Stockings were the first pro team, there were many amateur teams before them. Did they even wear masks back then?
In 1951, in the novel ‘Catcher in the Rye’ Holden puts his red hat on with the peak backwards because it looks good, but he doesn’t think he’s the first - He’s emulating the ‘Catchers’
Hence the name.
nm
The sniper on the TV series ‘SWAT’.
Not sure this really counts, but some styles of tricorn hat are ‘backwards’ with respect to other styles.
I also thought about bicorns/cocked hats; some people wore them skinny-end forward and some people wore them flat-side forward.