For those programmers coming from the Windows world, you might be interested to know that an IDE with a fully integrated GUI designer will be available shorty from the MonoDevelop project. Currently only C# language features are supported but there's support for Boo, Java, PHP and Python in the works. Thus far there has been no OSS IDE with an integrated GUI designer available for Linux. The closest was KDevelop which included Qt Designer as a plugin.

Here is the announcement: http://primates.ximian.com/~lluis/blog/pivot/entry.php?id=49