I meant the 1k5 current limiting resistor, which is rather high for a Pierce oscillator powered by 5V supply. You will normally find current limiting resistors in the range of some hundreds of Ohm, if at all.

In the following post I reported about calculations refering to Pierce oscillator.

http://www.8052.com/forum/read.phtml?id=79811

I found, that the current limiting resistor (in Robert's application the 1k5 resistor) is needed not only to limit drive level of quartz to sane value, but also to introduce enough phase shift (in combination with burden capacitors) to make the phase curve surely pass the -180° line. So, the resistor does two things: Current limiting and introducing phase shift.
But there's also a third advantage of this resistor: As it introduces the more phase shift and the more dampening the higher the frequency is, it helps to dampen oscillations on harmonics of quartz's fundamental frequency.

'Rp' (in Robert's application) is not a current limiting resistor, of course. Its purpose is to linearize the inverter to make it work like an 'amplifier', as Andy Neil elaborated. The actual value of Rp is rather uncritical.

Kai