Well. Mari has a bunch of natural white, alpaca-silk blend yarn - I don't know the weight, but swoooon! And she wants to know what kind of sweater she should knit with it. I think that's a very good question, and if I had a bunch of natural white, alpaca-silk yarn (yah - I wish), I'd be thinking Aran. Something cable-y like Jenna Wilson's Morrigan, from No Sheep For You. (Not a lot of images around, outside of Ravelry, but you can follow Morrigan's process on Jenna's blog). Or, if the yarn is bulkier than that, maybe Eris, also by Jenna Wilson, though that refers to "advanced cabling," which some might find a bit intimidating. Ooo! Speaking of advanced cables, what about Alice Starmore? Though that's out of print. Hmm.
For something simpler, I really like Juju's variation on Jen Chau's Cavern Cardigan. Mostly, though, I think I'd pick up a book on cables, like my copy of the Harmony Guide to Aran and Fair Isle Knitting (or maybe use this as an excuse to buy a new book on cables, hee!), and assemble something myself. Something fine and girly, because alpaca and silk sounds so refined.
Anyway, good luck with deciding, Mari, and let us all know what you choose!