Well that's not available yet, all messengers have this stupid thing that you can't chat with users of other messengers.  I have found scripts for windows messenger, so you install plus and then scrips for it. that gives you gmail on msn, and lots of other stuff.  When that feature will be enabled, and if it is for gtalk, you must look for it on gtalk pages - look for additional software or updates. gtalk is still in beta and it will be long time for them to do that, but i know that there is one messenger that you can use it to contact all the contacts from talk programs that your friends use.  Try to find if the script or application for gtalk exsist, it can any day. try here: http://www.google.com/talk/about_whatsnew.html
If you don't find anything write to my email and i'll send you link for the messenger that works with all talk programs, but i have to look into my favorites to find the name. 

Not that I am aware of.  You can talk between MSN and yahoo and there are some third party apps that can talk to many mesenger platforms.  But I am fairly sure you can't with gtalk