@richard: Since you’re not getting a more specific error message when you activate SEO Ultimate, it’s possible that PHP error reporting may be disabled on your site. You may need to contact your host to enable error reporting. (Since I don’t have a specific error message to work with, it’s hard for me to guess what the problem would be.) Also, what version of PHP does your site have?
Here are couple of general debugging ideas:
1. Try disabling other all plugins before activating.
2. Try reinstalling SEO Ultimate. To do this, edit seo-ultimate.php and change the “Version: 1.9” header to “Version: 1.8” (towards the top of the file). This will trick WordPress into thinking you have an old version. Then wait until WordPress says you need an update for SEO Ultimate, and then use the automatic plugin updater to install 1.9 again. (It’s best to use this method instead of deleting/reinstalling the SEO Ultimate plugin from the WordPress plugin admin because doing so would delete your settings as well.)
Let me know how it goes.