I don't know if this is an Akismet issue or a WP issue (or even a host issue); I've contacted the Akismet people with the problem as well.
When I use the "Plugin Editor" page of my Admin panel to try to edit akismet.php (this happens whether or not Akismet is currently enabled), I get a 503 error when I click "Update File," and the file doesn't get changed. It's not a truly general problem, because I can edit other plugins just fine. It's also not a glitch--I've tried this multiple times, over a couple of weeks now, always with the same result.
This isn't like the 503 errors involving Akismet that I've been able to find posts about--this isn't a 503 error produced on one of my public blog pages; it actually happens in the Admin panel itself.