How do I fix user/owner 99

Having upgraded to WordPress 2.7 on a few sites and been quite happily installing/upgrading plugins; I now come to upgrade a plugin but I cant, WordPress fails as it cant delete the files, but when I try to through ftp I also cant because it seems they are all owned by 99.

So, whats with this? Is it expected behaviour? Do I need to change something to prevent it recurring and more importantly how do I fix it?

I dont have ssh access to my webs erver.