I just read DragonPetter's link and it has a blurb which hints and what height is practical to measure with the pressure sensor based on change in resolution, no mention of accuracy though. But obviously with reduced resolution comes reduced absolute accuracy in a real sensor.
It also implies the equation z = (RT/gM).loge(po/p) holds all the way to space.
"The altimeter would actually work all the way to space (vacuum), and still deploy. The problem when you start getting above 40 or 50 thousand feet is the resolution goes down significantly. The change in pressure per foot gets less and less the higher you go. By the time you get to 100,000', you may have to drop 1,000' before the altimeter detects enough change in pressure to fire the apogee charge. So if you plan to use this at high altitudes, make sure you design the recovery system with high speed deployment in mind."