Ok, so not everything works out in knitter-land.
Pattern: Eowyn by Theresa Venning from Rowan 38.
Size: Just the one size.
Yarn: Some really fabulous Álafoss Lopi by Ístex. This yarn is both fabulous because it's a single ply bulky from Iceland, and because I bought two-and-a-half skeins of it from an op shop back in Australia. The pattern actually calls for a strand of Rowan Kid Classic and a strand of Kidsilk Haze held together, knitting at a gauge of 12 stitches to 10cm. Unfortunately, the lopi was not a good substitute, even though it does knit up at a similar gauge. It just made the armwarmers, particularly the seams, too stiff and bulky.
Needles: 7.5mm circular and a cable needle.
Start to finish: 19 October 2009 to 20 October 2009.
Comments: I knit these in the round, whereas the pattern calls for them to be knit flat but I really can't see the point of an extra seam. The construction is very simple - the armwarmers are just a tube with a cable knit into it, no shaping. The lace trim is knit separately and then sewn on to the tube (didn't find the instructions for this detailed enough - slip stitch or mattress stitch? neither worked well in the lopi) leaving a gap for the thumb if desired.
Given this, as far as substitution goes, you could actually make a version of these armwarmers with any yarn - just calculate how many stitches you need to cast on to create a tube of suitable circumference in the yarn that you want to use. Knit that tube to the length that you want, including a 12-stitch wide cable, or any other sort of cable/decoration you like. Then knit the lace trim to the required length, making it wider with a few more knit stitches at the straight edge if you want. Attach the lace and you'd be done.
Verdict: Just didn't work out this time around but I would still like to make these armwarmers, either out of the suggested yarn or something else more appropriate than the lopi, or just use the tube plus lace concept in a different weight yarn altogether.