Server Push vs Server pull in IIS

I was thinking of building a chat application in ASP .NET. I was thinking of eithe开发者_StackOverflowr using push or pull. I have found that pull is more scalable than push. But does any body know if the web chat messenger like gtalk, yahoo etc use pull or push ?.

Thanks.

Gmail and such are using comet. This is a push architecture.

There is a nice article on ajaxian describing how comet works, this technique is used to push data from the server to the client.

But I am really looking forward the WebSocket, I think it's gonna make a lot of things more easy.