MU 2.8.1 introduced a number of bugs unfortunately. Things like the upgrade notices were annoying but the My Blogs page not appearing for some users was really bad.
This alpha release fixes those bugs and more. Grab it from
http://mu.wordpress.org/wordpress-mu-2.8.1a-alpha.zip and try it out on a test server please.
You can also grab the code from svn at http://svns.automattic.com/wordpress-mu/branches/2.8/ but you'll need your WordPress.org username/password.
Also, in 2.8.1 the sitemeta functions (*_site_options) work more like the WP *options functions. The old behaviour was to run the value from sitemeta through stripslashes() but now it doesn't. See #1060 for more details.
I know this is a pain for plugin authors but it's better to make these options functions consistent across WP and MU. WordPress itself has dummy *_site_option() functions already so we really need to make this change before the merge.
Please test, if you find a bug, take a look at the timeline, upgrade to the latest 2.8 branch code (there's a zip file link at the end) and if the bug still happens file a bug report if there isn't one already.