As one can see in this excellent rant by Ilia there are different views on how this should have been handled (also read the internals ML if you need even more reading material), namely why PEAR was polluting generic names and why they didn't fix it right away so PHP could get the glorious name of Date.
As many have noticed 5.1.0 had some issue when it was released, like for starters it introduced a empty date class by default which effectively killed scripts that used PEAR::Date.
Heigl takes the side of the internals developers, that internal PHP should win out over PEAR on this issue. There's been a pretty large debate raging on about this one - and, even with the (pullback) release of PHP 5.1.1, it goes on as an issue to address in, say, PHP 5.2? (Or maybe they'll just wait until 6 and lump it all in...)
