I don't have any experience in saliva monitors, but have you checked into purchasing a Clearplan Fertility Monitor? I bought one on EBAY for a lot less than what you would pay in the stores. It's still fairly pricey, but seems a lot more reliable than traditional OPKs. I have yet to use it, but I've read a lot of positive reviews on them. I think that as long as your cycle is between 21-42 days you can use it with pretty accurate results. I think what it does is store your cycle information and after a few cycles, it gives you an indication of "low," "high," and "peak" fertility.
Hope I didn't break any rules with all that info... Sorry!
DS-Evan, 7/19/2003. Mild PE at 40 weeks