Has there ever been a perfect game thrown in professional baseball
during spring training? How many pitchers were involved (I would be
surprised if it was one pitcher) and when did this occur? Has it occurred more
than once? What about recent times (last 25 years)?

At least once:

http://www.canadianbaseballnews.com/archives/onekind.html

Here's a little more detail and some other perfect game facts:

http://www.angelfire.com/fl/yrag/perfect.html

Almost happened in 1997:

"Boskie pitched the first four innings and Julio Valera the next two,
with Todd Frohwirth working the seventh, Ken Edenfield the eighth and
Mark Eichhorn the ninth in a 15-0 romp against San Francisco March 16.
In fact, but for Boskie's wild pitch on a third strike in the third
inning, the Angels would have had themselves a perfect game."

http://www.usatoday.com/sports/baseball/sbbw5233.htm

Thanks Bowler! Great info. I searched on Google before but must not
have used the right keywords. This is great trivia info.

Just a few things.  

"The Elias Sports Bureau, could not track when the last spring perfect
game was, or if one exists"

So, I doubt anyone else would know.  The Elias Sports Bureau is
top-notch when it comes to baseball research.

Also as far as using the right search terms it sometimes just a matter
of luck and sometimes a matter of plowing through hundreds of web
pages (not my idea of fun).  I happened to use:

"perfect game" + "spring training" at (what else) www.google.com and
it was the second result, thankfully.

://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22perfect+game%22+%2B+%22spring+training%22+

Hope this helps.

Bowler-ga