Hi Paul,
I only have one duet at the moment for trial, but it has been enough for me to select them/slimserver as the music distruibution choice. the duet controller works well as is pretty wife-proof, base station is a little expensive but again works well. The limit to the number you can have on the network will only really be limited to the speed of the server that you put the slimserver on - which can obviously be scaled.
In regards to ipeng - interface is pretty BUT it is webbased talking as a html pluggin to slimserver and as far as I'm concerned too slow. I have to assume that when they finally write a native app this will solve the problem since the raw web packet management of the itouch is very effecient as can be demostrated easily when using it to control a webbrick directly (0 visible latency)
Have indeed seen airremote and registered with them when they annonced, anyone's guess (unless I've missed something) when it will actually be on the shelves and I'm hoepfull that the webbrick team are talking to the developers for integration as it should be a good link.
You don't have to buy a new itouch - it works as long as you upgrade to V2 firmware
Duets I would still be looking (as I did before) to source from the US when I travel there, although with the £ in free fall it may soon become a different story.
As of today itouch is still my favoured channel for room touchpoints, cost and speed are great (have to assume that the bugs, which apple have loads of, will be fixed, and native apps will appear) - there is also loads of rumours about a new lower cost range coming soon
hope this helps