Several pages on this site mention the use of Von Frey hairs in the diagnosis of allodynia. They provide a quick, simple, and cheap means of doing a clinical assessment in-office and I'm still a little surprised that they are virtually unknown, or at least rarely used. Basically, patients with the type of nerve damage to cause central pain usually have poor (absent or delayed) response to light tactile sensation and an increased response to anything above a certain threshold. 

Some physicians have written to inquire for more information about Von Frey hairs and where they can be purchased. Thanks to some readers that have written to PainOnline, I can provide a little more information.

Sources
Somedic (Sweden) offers a set of von Frey hairs, but they call it an Aesthesiometer. It is a case with 17 filaments mounted on plexi-glass handles. I haven't communicated with them directly, but I understand they cost roughly $650. 

I've heard that a company called Stoelting in Wooddale, Illinois also offers a set of handheld filaments, but I am not having any luck finding a listing for them. If anyone knows whether they are still in business, please let me know.

Guidelines
The following sources offer information on the use of Von Frey filaments. Thanks to Dr. Robert Dworkin, Director of the Anesthesiology Clinical Research Center, University of Rochester School of Medicine and Dentistry. 

The best all-around reference is:
Boivie J, Hansson P, Lindblom U., editors. Touch, temperature, and pain in health and disease: mechanisms and assessments. Seattle: IASP press, 1994.

You may also find information on general approaches to quantitative sensory testing in the following chapter:
Dworkin RH, Nagasako EM, Galer BS. Assessment of neuropathic pain. In DC Turk, R Melzack, editors, Handbook of pain assessment (2nd ed.) (pp. 519-548). New York: Guilford Press, 2001.

A recent study that also has very worthwhile methodological information is:
Pappagallo M, Oaklander AL, Quatrano-Piacentini AL, Clark MR, Raja SN. Heterogenous patterns of sensory dysfunction in postherpetic neuralgia suggest multiple pathophysiologic mechanisms. Anesthesiology, 2000;92:691-698.

Other Information
For additional information on allodynia, see Allodynia and Common Errors in Evaluating Allodynia.

Below are a few links I have located with a little information about Von Frey hairs. 

Max von Frey: Pain Spots
A brief paragraph on Max von Frey, the inventor of Von Frey hairs. 

Stimulus Experiment using Von Frey hairs with Pre-Term Babies 
Not about Central Pain, but a fairly good basic discussion what Von Frey hairs are and how they may be used. 

Child's experiment 
Scroll down to "Become a Neurologist," grades 4-12.
Ironically, one of the better online discussions of Von Frey hairs is on this page, which offers several basic medical experiments meant to interest children in medical science.