FYI, GPLv3 has an official way to include attribution, but that's not relevant unless Geeklog moves to it.
With that aside, here's a shocking revelation - PHP-Nuke have been claiming for years the GPLv2 situation is fixed by renaming "powered by" to "copyright by":
http://phpnuke.org/modules.php?name=News&file=article&sid=4947 (note it continues in a follow up link).
Let me sum up: GPLv2's section 2(c) dictates interactive copyright notices must remain. Not liking attribution, GPL obviously meant for these notices to just mention the GPL license. But an official from GNU assured (not legally, just conceptually) the developer of PHP-Nuke there's no reason not to include the actual word "copyright" as well (e.g. "copyright by Geeklog, this is a GPL software...", etc.). The official was aware it's a web app and not a binary, but he also consulted with another official.
The link claims the ticket they opened in GNU's tracker is listed in GNU's archive. I found no such feature as "archive" in GNU's site. Nevertheless, it seems so simple I'm surprised it has no mass usage in web apps.
So what do you think? Note most of the follow up link is the developer defending himself against censoring masses of users who wanted to punch him for finding this fix (after they got used to casually manually removing the attribution of PHP-Nuke).