Tiger officially supports Macs sold in 1999 (and with a simple "hack" even older). Leopard will most definitely support the current Mac mini, and I bet the next release after that will too. You have to remember, as of right now, 100% of in-use Macs are "obsolete" PowerPC machines. Even if Apple sells 5 million Intel Macs by the end of the year when Leopard comes out, there will still be many many more PowerPC Macs out there in use. Apple (and other developers) would be shooting themselves in the foot if they didn't support all those Macs.
PowerPC Macs will continue to run new software for sometime. Developers are being encouraged to prepare and release universal binaries. Assuming they're using XCode and making the modifications necessary to make a universal binary, it's no extra work to continue supporting PowerPC. Just one extra checkbox in XCode, and a slightly longer compile time. Expect no problem running software with a PowerPC machine for several years to come.