I heard there is a new law coming that will allow to keep the cellular
phone number when switching carriers, from SprintPCS to TMobile for
example. Right now I have sprintPCS phone and I want to switch to
T-Mobile because they offer GSM, but the only thing keeping me from
doing that is that I must loose my current phone number. Can someone
tell me when this law will take effect or if there is another way of
switching carriers without a loss of phone number.

The 'law' you are referring to (it would actually be an FCC
regulation, most likely) goes by many names, but it's easiest to find
information on it by it's technical name - Cellular Telephone Number
Portability. As the commenters posted below, this law is very far off
as the cellular lobby is fighting it tooth and nail. If you are
considering switching cellular providers, I would do that now, as you
aren't going to be able to switch and keep your number any time soon.
The FCC set a deadline for this of November 2003, but they've pushed
that back twice already, so I wouldn't hold your breath on being able
to switch even then. Some of the links below may be useful to you to
help you understand what number portability is and how it works.

Thanks for the great question!

/gleffler-ga

Resources:
http://www.fcc.gov/cgb/consumerfacts/numbport.html
http://news.com.com/2100-1033-944168.html?tag=rn

Search:
://www.google.com/search?q=cellular+number+portability

thank you all.

I actually heard about this on the Clark Howard radio show.  You
should go to the show's web site (www.clarkhoward.com) for more info. 
Basically, he said that this law is pretty far off, if it even ever
does become law.  Phone companies don't want it, because without it,
people like you are "locked" into their service.

http://clarkhoward.com/library/tips/wireless_phones.html

Check out the July 18, 2002 entry.

Gotta love Clark.