KNX is a solution which works without a computer. That is correct. If you want to utilize iPod Touch to control your KNX network, you have two choices:
1) Use LinuxMCE - This means, your iPod Touch tells LinuxMCE to do stuff. LinuxMCE talks to the IP/KNX gate, which in turn talks to the KNX actuator.
2) Have a KNX capable software running on your iPod Touch - I do not know of any, but that does not mean nothing exists.
Centralized wiring is always better imho, than decentralized. Or a mixture of both, which is what I have.
What you should think about is, whether or not LinuxMCE fits into your scheme of things. If it does, I would use IR blaster/receiver which are LinuxMCE compatible, regardless of KNX vs. ZWave, i.e. either a regular GC-100-6 for example, or, if you have MDs scattered around, USB-UIRTs.