I think it is great the response to this change. I just found out about it 2 days ago (and I am from Argentina) and I was surprised. It is going to be lots of problem for companies that deal with Argentinean time. I am so happy I don't have to worry about it :p
Sad, that goverments make political decisions on a blink that can be so bothersome

Great Response Time, mostly because the regulatory law was made on December 28.
Sadly i have a little issue with my exchange's 2007 servers.
The have the new time, but the OWA webpage don't show the new timezone :(
Do i have to do any other step in order for the new zone to appear in OWA?
This can be because i didn't reboot the servers after the change?

Thanks in Advance,
I really enjoy the Exchange Product and your Blog.

Pablo Halamaj

Pablo,
It is not necessary to restart the server
Specifically for OWA's option page you have to restart the "World Wide Web Publishing Service" service for the new time zone to show up in the listing.
Other services that should be restarted after you import the registry key, are "Microsoft Exchange Mail Submission” and “Microsoft Exchange Transport”.  

Hello Latam Friends,
I read the comment from Pablo Halamaj above. I have the same problem with OWA 2003 not displaying the new time zone. I have restarted "IIS Admin Service" a couple of times however I still don't see the change. Do I need to restart the whole box?
Thanks,
João Paulo H.

Don't worry about the previous question. I found the answer here: http://blogs.msdn.com/mthree/archive/2007/12/29/argentina-122907.aspx
Impact for Outlook Web Access clients in Argentina

The new time zone “(GMT -03:00) Buenos Aires” created following the recommendations on section “Updating Windows Servers and Desktop Operating Systems” will not be available on the OWA (Exchange Server 2000 and Exchange Server 2003) time zone options at this time. In order to have this new time zone available in OWA, there is a dependency on an index key which is not currently defined. We expect this to be defined over the next months.
After you configure Windows clients and servers to use the new time zone “(GMT -03:00) Buenos Aires”, OWA users using “(GMT -03:00)  Buenos Aires, Georgetown” on the OWA time zone options, will receive the following warning when accessing their inbox: Use the Options page to set your local time zone. When you open a message in OWA, it will be displayed with the incorrect time during the DST period. This is related to the CDO components in Exchange and there is no workaround for this behavior at this time.
Cheers!
João Paulo H.

Brazil also changes DST crazily every year, and we have a few problems with our calendars when working with people from other countries.
Windows doesn't change the time properly when the government come up with a random date for DST, and we have to change it in our computers manually. Then all calendar, meetings, etc go a little crazy :) We never know when our meetings are... maybe now, may be an hour ahead...