I did the autoupdate to 1.2 this morning. It said it completed, but at the top, it said this:
There is an error. Please report the message and full backtrace in the Piwik forums.
Warning: unlink(C:\Websites\GibsonTechnologies\piwik/tmp/latest/latest.zip) [function.unlink]: Permission denied in C:\Websites\GilsonTechnologies\piwik\plugins\CoreUpdater\Controller.php on line 110
Backtrace -->
#0 Piwik_ErrorHandler(2, unlink(C:\Websites\GibsonTechnologies\piwik/tmp/latest/latest.zip) [function.unlink]: Permission denied, C:\Websites\GibsonTechnologies\piwik\plugins\CoreUpdater\Controller.php, 110, Array ([archive] => Piwik_Unzip_ZipArchive Object ([] => ZipArchive Object ([status] => 0,[statusSys] => 0,[numFiles] => 2283,[filename] => C:\Websites\GibsonTechnologies\piwik\tmp\latest\latest.zip,[comment] => ),[filename] => C:\Websites\GibsonTechnologies\piwik/tmp/latest/latest.zip),[pathExtracted] => C:\Websites\GibsonTechnologies\piwik/tmp/latest/,[archive_files] => Array ([0] => Array ([filename] => C:\Websites\GibsonTechnologies\piwik/tmp/latest/piwik/,[stored_filename] => piwik/,[size] => 0,[compressed_size] => 0,[mtime] => 1299254146,[index] => 0,[crc] => 0),[1] => Array ([filename] => C:\Websites\GibsonTechnologies\piwik/tmp/latest/piwik/tmp/,[stored_filename] => piwik/tmp/,[size] => 0,[compressed_size] => 0,[mtime] => 1299254134,[index] =>…#1 unlink(C:\Websites\GibsonTechnologies\piwik/tmp/latest/latest.zip) called at [C:\Websites\GibsonTechnologies\piwik\plugins\CoreUpdater\Controller.php:110]#2 Piwik_CoreUpdater_Controller->oneClick_Unpack() called at [C:\Websites\GibsonTechnologies\piwik\plugins\CoreUpdater\Controller.php:62]#3 Piwik_CoreUpdater_Controller->oneClickUpdate()#4 call_user_func_array(Array ([0] => Piwik_CoreUpdater_Controller Object ([] => ,[] => Array (),[] => Array (),[] => Array (),[] => CoreUpdater,[] => ,[] => ,[] => 0,[] => Piwik_Site Object ([] => 0),[pathPiwikZip] => C:\Websites\GibsonTechnologies\piwik/tmp/latest/latest.zip),[1] => oneClickUpdate), Array ()) called at [C:\Websites\GibsonTechnologies\piwik\core\FrontController.php:125]#5 Piwik_FrontController->dispatch() called at [C:\Websites\GibsonTechnologies\piwik\index.php:60]