Laceweight yarn is typically anything over 18 WPI (wraps per inch). Once you start getting into 40+ WPI, _definitely_ cobweb weight!
Jamieson & Smith has "Shetland Cobweb" yarn that is stated to be 32 WPI, with 380 yards in 25 grams (25 grams = 0.88 ounces; so it would be about 430 yards per ounce -- a pair of socks is typically about 400 yards, in 4 ounces!).
A great parenthetical comment shows up under the WPI entry at everything2.com, "(keep in mind cobweb is to regular lace pretty much as fingering is to worsted, and often even lighter.)" So right!
(based on a posting by me this day on spindlers)