The exact same version will be added to the launcher on Wednesday April 4, at 15:00 CEST (06:00 in California, 09:00 in Florida, 22:00 in Korea, and 23:00 in Sydney).
That's 14:00 BST, for us people in the UK!
YAMS won't be updated straight away, we as always have to wait on CraftBukkit + plugins so I suggest you do not update. However should you wish to go and play 1.2.5 without us, please make a copy of your bin folder or the minecraft.jar file so you can come back and play on YAMS if you get bored!
Bin folder located at the following locations ( “~” = username):
Windows 2000, XP, and Server 2003: C:\Documents and Settings\~\Application Data\.minecraft
Windows Vista, Windows 7, Windows Server 2008 and R2: C:\Users\~\AppData\Roaming\.minecraft
Mac: ~/Library/Application Support/minecraft
If you forget to do this, you will need to use a program like McNostalgia to come back, however bear in mind this takes a while to update.
As always I will keep you updated on the latest news of updating, if you have any concerns relating to YAMS and 1.2.5 please feel free to ask in here and I'm sure someone will be able to answer you.