In reply to PontiusPirate:
define what you mean with ski touring...
day tours, no glacier stuff.
Something like 20 to 25 liters is enough (skins on the way down, perhaps gloves a thin belay jacket and some snacks plus water). Just make sure the ski-carry is good (more often than not, diagonal carry) and that you can also attach yer brain-lid (if you're using such).
Perhaps something like Arva patroller 24
http://en.arva-equipment.com/fiche/patroller-24.html
Haglöfs Skra 26
http://www.haglofs.com/en-US/products/backpacksbags/ski/unisex/skr%C3%A5_26...
And numerous others.
If you're definition also includes glaciers, rappelling and all that fun... Get something in the 40 to 50 liter region. And best stick to models that are made for ski touring / -mountaineering. Like Haglöfs Rand 50 and so on.
The odd 30 liter models are a bad compromise. Not big enough for proper stuff, yet too big to be used on mellower tours (as in you'll pack too much crap with ya). Unless yer hudge, when simply even the smallest belay jacket is around 25 liters in volume.
Of course, if you don't know what to do, then the odd 30 liter model will be fine. But as mentioned above, you can't get all the stuff with ya or you'll bring too much.
My pick for the pack has been Arva Rescue 27 (old model). Which seems to be ok, perhaps a tad big for non technical touring and I can just about squueze in some technical glacier stuff in (but not the rope).