I played around with a Bosch Sensortec BMP085 pressure sensor a while ago. It has a digital output, not analog. The datasheet claims a noise level of 0.03 hPa (standard deviation of 10 readings) in the "ultra high resolution mode".
My experiments with two of these units in the same sealed jar, with very slowly changing temperature, showed a p-p noise level around 0.15 hPa and the two sensor readings differed by about 0.35 hPa, giving me a feeling for the accuracy that might be achieved. Can anyone suggest a (small, inexpensive) pressure sensor which does better than this (lower noise)?
Here are some graphs and data from the BMP085: http://jeelabs.net/projects/cafe/wiki/Pressure_Plug_measurements