Other people can find the PTR client inside of the folder "C:\Users\Public\Games\World of Warcraft Public Test" in Windows Vista and "C:\Program Files\World of Warcraft Public Test" in WindowsXP. MacOS X users can find the files in "MacintoshHD::Applications::World of Warcraft Public Test".
This important bit of information came from blue poster Maaven on the official forums today. With Patch 3.0.2 being released tomorrow, the PTR client is useless and came be safely removed. But be sure that you remove it the way Maaven suggests.
WoW Insider will have a lot of 3.0.2 coverage tomorrow. We'll also be keeping the light on tonight and will get you the patch notes as soon as they appear.