WordPress 2.8.1 fixes some securty bugs and other issues as described below:

• Certain themes were calling get_categories() in such a way that it would fail in 2.8. 2.8.1 works around this so these themes won’t have to change.
• Dashboard memory usage is reduced. Some people were running out of memory when loading the dashboard, resulting in an incomplete page.
• The automatic upgrade no longer accidentally deletes files when cleaning up from a failed upgrade.
• A problem where the rich text editor wasn’t being loaded due to compression issues has been worked around.
• Extra security has been put in place to better protect you from plugins that do not do explicit permission checks.
• Translation of role names fixed.
• wp_page_menu() defaults to sorting by the user specified menu order rather than the page title.
• Upload error messages are now correctly reported.
• Autosave error experienced by some IE users is fixed.
• Styling glitch in the plugin editor fixed.
• SSH2 filesystem requirements updated.
• Switched back to curl as the default transport.
• Updated the translation library to avoid a problem with mbstring.func_overload.
• Stricter inline style sanitization.
• Stricter menu security.
• Disabled code highlighting due to browser incompatibilities.
• RTL layout fixes.

But, most important (?), it’s fully compatible with Triqui MochiAds Arcade theme and plugin with automatic upgrade from your dashboard.