Fan theory - None of the stuff from the past actually happened ...

Interesting theory... And some things might seem to support your version (no one knowing who he is despite his successes, for instance).
But then again the 'now' isn't really now though, is it. Meaning the world in 'now' is still a pretty huge place, foremostly with no internet to tell everybody everything about everybody.
The "no one" who doesn't recognize him are actually only three people. One who seem not to really listen (nurse), one who instantly takes him for a simpleton (man in hat) and the old lady - who enjoys his story but doesn't really believe him, until he shows her the magazine cover.

But, as this theory of yours make the story a lot less "magical" (for want of a better word), I'll chose to believe in the actual narrative. :-)

....but I haven't read anything anywhere to indicate that this is the case.

Thoughts?

Except for the fact that he showed that women the cover of a magazine that proved he was the owner of the Bubba Gump Shrimp Corporation. Him and Lt. Dan were on the cover. Unless you think that old lady was not real either.

You also have to keep in mind that the "present day" in the movie when Forrest is sitting on the bench is 1980 or 1981. Information was not as easily accessible nor as quickly updated as it is now. There was no internet or cell phones, and computers were just starting to become mainstream (remember the "fruit company" in which Lieutenant Dan invested them in?). It is quite plausible that no one recognized him, especially since, as one poster pointed out, he was in Georgia, not Alabama.

My interpretation of the film was that Forrest is someone who tells these tall tales (and pretends to be dim)in order to get people to let their guard down before he shoves them into his van, locks them up in his basement and ultimately skins them and wears their skin over his while dancing.

Jenny did show him all the magazine articles of him running

My iMDB profile http://www.imdb.com/name/nm4297325/?ref_=fn_al_nm_1

I met a Canadian BILLIONAIRE last year. I had NO IDEA who he was until I looked it up on the internet.

In 1982 (the year Jenny died) there was no internet.

Consider this;
Lt Dan was running the Bubba Gump Shrimp Co.
When Gump was running across America ,he had long hair and a thick beard. Fairly unrecognizable.
Some people may not have been subscribers to Fortune Magazine when Gump was on the cover.
I wouldn't know the identity of everyone who won the Medal of Honor, do you?
I couldn't name one member on the USA's last ping-pong team, do you? Or the men's gymnastics team, for that matter.

It is entirely possible that a billionaire simpleton like Forrest Gump could keep a low-key existence in Greenbow, Alabama.

Getting the Medal of Honor doesn't make you a household name (unless your a die hard military buff). Neither does being a "Ping Pong Champion" or being the owner of a shrimping business. It's not like he was a huge movie star or singer.