Thus far the only software that hasn’t behaved as expected when recompiled with the -O3 -march=pentium3 -pipe -ffast-math -frame-omit-pointer CFLAGS has been Firefox. I had to recompile it last night with the old, safe CFLAGS because the “overoptimized” version just spun circles in the background, never appearing and never doing anything but eating processor time.
I don’t have any theories on why it didn’t work, although it’s possible that it’s just on one machine that it seems to fail. Of course, it could be that Firefox’s code is so gnarled and bloated that overoptimizing it causes a collision of realities — like the immovable object and the irresistible force. Gee, I didn’t know I had tempted destruction so blithely. 
Since habving to retreat to gutsy to get my usb working properly, I recently tried my hand at building firefox and was surprised at how easy it went (well, with instructions anyway: http://dbaranski.wordpress.com/2007/09/19/building-firefox-on-ubuntu/) Apart from rather crappy anti-aliasing it works most of them time. Can I just ask if you know of a resource/explanation page for the optimize flags? The mozilla page just discouraged their use ‘if you don’t know the code base very well’ (or something)…