When installed on 64-bit system does Leopard runs automatically in 64-bit mode?

On AMD systems there is no support for 64-bit. However, things might change when Snow Leopard is released.

But it works on Intel machines in 64-bit mode?

Adobe CS4 will be 64-bit ready, so I'm wondering how it will use resources on Hackintosh...

And btw is there a way to see in which mode Leopard is running currently?

Yes 64 bit mode works on Intel, I am running it at the moment, a good way to test it is by trying to run a 64 bit app, such as geek bench.

Simple way is to 'get info' on Chess, if you are running 64bit it'll give you the option to run it in 32bit mode. This is how it displays on my MacBook Pro:

http://www.grabup.com/uploads/21ddce...d71548121d.png

Thanks. As said before it works only in 32-bit mode on AMD and that's what I see via Chess and Geek Bench.

the Windows Vista version is 64 bit however the mac version is 32 bit only

wikipedia: "OS Windows (32-bit; 64-bit exclusive for Windows Vista) and Mac OS X (32-bit)"
betanews article: "Mac OS X users probably won't get 64-bit support until CS5"