There's a thread on GL here when I asked about it a couple of years ago -
http://www.guinealynx.info/forums/viewtopic.php?t=32261
Quotes from research include...
"This product exhibited potential to produce dermal sensitization in guinea pigs" http://www.vpl.com/msds/adams_ftmist_msds.html
"In animals, large oral, inhaled, or topical doses of pyrethrins have produced CNS excitation, incoordination, tremors, seizures, and death." McEvoy, G.K. (ed.). American Hospital Formulary Service - Drug Information 2000.Bethesda, MD: American Society of Health-System Pharmacists, Inc. 2000 (Plus Supplements)., p. 3204
"Non-systemic insecticide with contact action. Causes paralysis initially, with death occurring later." - Tomlin, C.D.S. (ed.). The Pesticide Manual - World Compendium. 10th ed. Surrey, UK: The British Crop Protection Council, 1994
It was something I looked into a long time ago when I was looking for shampoos to buy for my piggies - I was told in several places I shouldn't use pyrethrin based shampoos or sprays and couldn't find out why. A few studies show no negative effects but, on the other hand, there is literature everywhere that says humans should avoid prolonged exposure (and wear PPE when administering it to large animals), it can cause neurological damage to other rodents and have effects on the litters of pregnant animals. The link you gave above, Niki, even says that certain extracts can cause allergic contact dermatitis in guinea pigs (though refined extracts do not) - how do we know which one is used in the cheap shampoo from the pet shop when all it says is "pyrethum extract"? Call me paranoid or overdramatic but I wouldn't use it on a child with headlice so I wouldn't use it on my guinea pigs. I, for one, don't want to chance it.
Once again it's down to personal choice (similar to how I feel about the likes of shavings) but it's something I've chosen not to risk, especially having a pig who has had skin problems in the past. It's also something my vet won't sell or recommend but they could't give me a reason as to why - they simply said (which I think sums it up) that there is better and more effective you can buy now.