For those with enough interest to keep with this, I've searched my butt off to find out that there are 3 patents on this stuff, 3 abandoned applications, and one rejection:
US 5,280,864, Jan 1994, which was a continuation application of an abandoned application 07/521,992 (May 1990), which itself was a continuation application of an abandoned application 07/031,157 (March 1987), which itself was a continuation application of an abandoned application 06/919,647 (October 1986). This appears to be a current patent.
US 6,098,924, which was patented on 8th August 2000 but expired due to non-payment of renewal fees.
US 6,347,766, which was patented on 19th February 2002 but expired due to non-payment of renewal fees.
Application US20060065789 (10/928,847), which has been rejected.
These numbers will be enough for you to pull off your own copies of these patents off of pat2pdf.org , or whichever patent download site you favour, should you be so inclined.