Though Devel seems to work fine in 1.5.0.1, the 1.4.2.0, 1.5.0.1 and 1.5.1.0 app/code/core/Mage/Core/Block/Template.php files need to be diffed, and any changes updates carried across to app/code/local/Mage/Core/Block/Template.php
-- UPDATE --
Check all version mentioned in title and diff their Template.php files.
Though Devel seems to work fine in 1.5.0.1, the 1.4.2.0, 1.5.0.1 and 1.5.1.0 app/code/core/Mage/Core/Block/Template.php files need to be diffed, and any changes updates carried across to app/code/local/Mage/Core/Block/Template.php
-- UPDATE -- Check all version mentioned in title and diff their Template.php files.