Likely never.
Snow Leopard is a $29 upgrade to Leopard, but it requires a Mac with an Intel processor (newer than 2005) and 1 GB of RAM.
The App Store is part of the OSX System software. If you don't have it you can update to a version that has it, which includes Mac OS X 10.6.6 "Snow Leopard" or later.
No because the Mac App Store requires Mac OS X 10.6.6 which is not supported on the old G3/G4 iBooks.
To do this, you will need to install 10.6 (Snow Leopard). A Snow Leopard disc can be purchased online, or at a local computer store. Once this is installed, you can purchase OS 10.8 (Mountain Lion) from the App Store that is built into Snow Leopard, and download it directly from there. Interestingly enough, Apple no longer releases their operating systems on discs, and instead sells them exclusively on the App Store. Cheers!
Mac OS Leopard
If you didn't know yet you have to have 10.6 (Snow Leopard) now as Apple is going away from the Apple store and everything is sold through the Mac App store. You have to have 10.6 installed in order to get the App Store application in order to access the Mac App Store. All of Apples software is now only downloadable from online. No more DVD/CDs.
Mac OS X Lion, it is available on the mac app store
You have to buy Mac OS X 10.5 Leopard from the store - you cannot download it from anywhere (that would be illegal).
Providing a Mac mini is running Mac OS X 10.5 (Leopard) it can be used to make iPhone apps. Apple provides separate versions of the iPhone Software Development Kit for Leopard and Snow Leopard. The SDK requires the Xcode development tools which are included with the Mac OS X installation disc.
Currently, it's $129 dollars (Mac OS X Leopard). Once Snow Leopard is available (August 28), it will be a $30 dollar upgrade from Mac OS X Leopard.
Mac OS X 10.6 (Snow Leopard) and all earlier versions of Mac OS X will support networks.
Yes, Mac OS X 10.5 Leopard ships with Java support.
To upgrade from Mac OS X 10.4 (Tiger) to 10.5 (Leopard) you will need to buy a boxed version of Mac OS X 10.5. If you have a Mac with an Intel processor you can go straight to Mac OS X 10.6 (Snow Leopard) which is a cheaper upgrade option.