All of a sudden I see the word "heyz" in spam emails, in user groups,
etc.  A Google search of "heyz" yields zillions of hits.  It's used in
a greeting sense, like "hey".

My question: where did this word come from?

I can offer a guess.

The A and Z keys are next to each other on a standard US keyboard. 
Maybe a few people fat fingered "heya" and it became an internet
trend.  I bet it's just another d00d speak term used by internet
gamers and in forums.

Basically d00d speak is intentionally misspelling words or replacing
letters in a word with similar looking numbers or characters.

All quite annoying if you ask me...

Another guess from the peanut gallery:

Up north ... hmmm... darnit, I'm not a southerner anymore...  they use
the phrase "yous guys" in strong opposition to the term "ya'll".  The
strong Z sound at the end of both of the words "yous guys" might bleed
into the greeting word hey when saying "heyz yous guys" never to be
confused with "howdy ya'll" which I must say I'm very fond of the
latter being a former southerner myself.

The only times I've heard the word "heyz" in spoken "English" were in
that phrase "heyz yous guys".

A young Internet pal of mine ends all his emails with 'Laterz',
meaning "See you later." Trendspeak seems to be pluralizing things
with a 'z' these dayz.