LeeAnn posted a comment asking about yarn substitutions for the Roped Shell in the Summer 2008 IK, and as I had been thinking about this subject, I decided to stick my neck out and offer a few possibilities (bear in mind that unless otherwise noted, I do not have personal experience with these yarns, and am basing my opinion mainly on fiber content and yarn weight/gauge). Remember that swatching is very important when switching yarns, and that you may need to try several different needle sizes to get the correct gauge. I do feel strongly that since the Filatura di Crosa Luxury (Roped Shell) and Tilli Tomas Fil de la Mer (Wakame Lace Tunic) are both very expensive ($18-20 per skein), suggesting more reasonably priced alternatives is something somebody should do, and since that somebody is not IK (ahem), I guess next in line would be me. I will say that I felt both of these yarns were excellent choices for their projects; the Luxury drapes very nicely, which is important for the look of the Roped Shell (and much less yarn is required for the shell than for the tunic), and I really love the subtle color variations and the texture of the Fil de la Mer. So if you can afford them, great. If not:
Roped Shell: LeeAnn had asked if I thought KnitPicks Elegance might work. I think KnitPicks Shine Sport would be a better choice, because the gauge is closer to the Luxury, and although the alpaca/silk combo in the Elegance might drape nicely (alpaca tends to have less resilience than wool), I think the modal/cotton in the Shine Sport would probably also drape quite well. Another alternative which is still rather expensive is Louet Euroflax sport weight, 100% linen. This is around $21-22 per skein, but each skein is 100 grams (or you can buy 1/2 lb cones), meaning it ends up costing around half as much as the Luxury or Fil de la Mer, and I have swatched with it for other projects, and like it a lot. Other possibilities: Classic Elite Cotton Bamboo (around $8), Reynolds Rise & Shine (cotton, $7-8), or Reynolds Saucy Sport (cotton, $3-4).
Wakame Tunic: KnitPicks CotLin might work nicely for this. Like the Shine Sport, it is very economical at $2.49 per 50 gram ball! Classic Elite Classic Silk (cotton, silk, nylon, ~$8), Ella Rae Silkience (cotton, modal, silk, rayon, $5-6), RYC Bamboo Soft (bamboo, ~$9) and Elsebeth Lavold Hempathy (hemp, cotton, modal, ~$6) are other possibilities; I have some Classic Silk and Hempathy in my stash and have swatched with both for other projects.
I haven't included any wool, alpaca or cashmere blends; certainly you could use one of these for a dressy-ish cool weather layering piece (the shell) or a more all-seasons tunic, but I would recommend keeping wool and nylon content on the low side for a nicely draping fabric.