Just to add to the many that have this issue, here goes my post.
Installed WP 2.2.2 clean
Imported DB from WP.com, which was hell itself
Fixed up all the pages, chose a theme, uploaded plugins, all working well.
We were using the demo mode plugin so the site appeared inactive to visitors.
After having every thing setup just the way we wanted, decided to open the blog up to the general public today.
We deactivated the demo-mode plugin, and within a few hours, we get the usual
Error establishing a database connection
This either means that the username and password information in your wp-config.php file is incorrect or we can't contact the database server at localhost. This could mean your host's database server is down.
And it randomly happens, such that the blog may work properly, where as the Dashboard may just show this error in parts...or sometimes it gives this error for the entire blog.
I checked and re-checked the wp-config file, nothing wrong, as it randomly works! All plugins disabled so no chance of anything but WP 2.2.2 running.
Upgraded to 2.2.3 in the hope that it might be a bug of some sort, still the same issue.
And it cannot be the host, as 3 other WP blogs running on this very hosting account have no issues whatsoever, not to mention a VBulletin forum.