As far as I understand it is allowed to remove that from the footer from wordpress point of view, that said it is generally a good idea to show support for wordpress by keeping it there.
As long as you ONLY remove the line that contains "Proudly powered by WordPress" from the template file "footer.php", it should not break anything.
WordPress is released under the GPL, and you can therefore do whatever you want with it as long as if you release the modified code, you release it under the GPL too.
WordPress is released under the GPL, and you can therefore do whatever you want with it as long as if you release the modified code, you release it under the GPL too.