It was discussed at the last couple year's Food Allergy Network Conference.
Not all labs offer the test though I *think* Smith Kline Labs do offer it.
My allergist ordered the test for my son but I haven't taken him yet to get
blood drawn.

"Relationship between food-specific IgE concentrations and the risk of
positive food challenges in children and adolescents".  Sampson, H. and Ho,
D.  Journal of Allergy and Clinical Immunology, October 1997, volume 100,
Number 4

An abstract of the study is available on Medline.  The full text article
discusses its reliability and positive predictive value and negative
predictive value.

The CAP-RAST system is more sensitive than a traditional RAST and can more
quantitatively report IgE levels (It only works for milk, peanuts, fish and
egg right now.  It doesn't work for anything else).  We are going to do
periodic CAP-RAST tests (maybe one a year or so) to see if the levels are
dropping, which may indicate the milk allergy is being outgrown.

Lynda