I am using Rowan Felted Tweed in the colorway "sigh". I bought this yarn on ebay specifically for this sweater after I read on Allison's blog http://allison.knitsmiths.us/ that this is the yarn that she used, but then when I got her book "Charmed Knits" www.amazon.com/dp/0470067, I found that the pattern was writted in a heavier weight yarn. I have never tried to adjust a pattern to use a smaller guage yarn, so I was very dissapointed that I was not going to be able to start the sweater.
Recently, I Allison on Ravelry and I wrote her with my dilema. She wrote me back :) (I have been following her blog for awhile now and I felt like I was writing to a celebrity). Anyway, she directed me to an older post on her blog where she had written the pattern in this very yarn! http://allison.knitsmiths.us/pattern_weasley.html Needles to say, I cast on to my size 5 needles right away!
ETA: My links above aren't working. If anyone reading this can tell what I am doing wrong, please let me know.