Posting in the Magento forums has been disabled pending the implementation of a new and improved forum solution which should better serve the community.
For new questions please post at magento.stackexchange.com, the community-run support site for the Magento community. We will be providing updates on the new forum solution soon. For questions or concerns please email firstname.lastname@example.org.
I used Magento Connect to upgrade to 1.2, and was having problems with the shipping methods in the checkout process not being available. To fix this, I tried redoing the upgrade using magento-core/Mage_All_Latest, and now all I get is a 500 Internal Server error trying to get to the frontend / backend or downloader.
Ok, I figured it out. After running the magento-core/Mage_All_Latest the index.php permissions were set to 666. I reset that to 755 and now the site is working again. I’m still having the problems with the shipping modules in the cart not working, so on to that now.