I totally agree with Silver, for me, it would be the gift cert.; Depending upon the $ amt., a nice medium size tote for knitting with a gift cert would be perfect for me...lol!
Seriously, a knitting book would be wonderful; or a cool pattern of something that you know that she likes to knit with the yarn for the pattern.
Knitting tote with various accessories; find out if she 'needs or wants' any certain needles (certain sizes of dpns or circs) and drop those in the bag.
Of course...yarn...we all love yarn!!
The cd rom from this website....EXCELLENT GIFT; a good friend gave me one....hugs to you, you know who u are
If she hasn't started organizing her patterns, etc; a nice 3 ring binder and a box of paper protectors and subject dividers.
Electronic row counter (love mine!!)
Out of all these things that I have listed; (other than the gift cert. of which some people don't like to give, my daughter refuses to give gift cert., she say's it's not very personal!) I would have to say a nice knitting book would be my #1 answer. The Knitters Bible is excellent; I just got it a couple of months ago and I have been knitting for a year...wish I'd had it the entire time...love it! The list of books are just endless!
Have fun shopping :D