I'm now running 10.6.5 on an iMac all seems to work great.
Also, I have a Canon printer connected via USB to an old Airport Express with the printer being shared by the iMac and my iPhone 4 can print to it. I've been able to do this for ages, in fact I could even do it on my old Edge iPhone too!
Now, I don't have iOS 4.2 GM or anything like that - but I do have a great little app called Air Sharing, which [in the full version] has the option to print to shared printers.
So my point is... if these 3rd party guys can do it - why not Apple too? I guess I'll wait till 4.2 hits my iPhone to see if Apples Air Printing is there or not... and if it isn't, then I'll still be able to print from my iPhone, so no bother.