For the G4 and Intel optimized builds of Pd-0.39.2-extended-test7 on Mac OS X, sqrt~ only returns -inf or inf. (Does this affect other platforms?) This is probably related to the auto-vectorization. This is not a problem on Pd-0.39.2-extended-test7 that was built for G3s.
Check the sqrt~-help.pd file for a test patch.
sqrt~ of the signal 9 returns -\infty, hence not 3 as expected. All tried int signals greater then zero returns -\infty. To test this, see either the attached patch (which is very close to the help-patch for [sqrt~]) or the help-patch for [sqrt~].