jb, I cannot reproduce this problem.
Did a clean compile of php 5.1.2 with lsapi 1.8 and overwrote an existing installation of php 5.1.2 with lsapi 1.7 on a live server. No problems.
Perhaps downloading a clean php source and recompiling from scratch will help. I have had problems in the past where even make clean did not clean out all the files from prior config.