(also applied to version 2.9.2)
Setting siteurl in wp_options to '' (the empty string) results in problems loading the blog/admin which is expected.
The error message however is rather misleading
The given errors are:
"Error establishing a database connection"
"One or more database tables are unavailable. The database may need to be repaired."
Going through the repair/optimize process is successful and yields no error, which left me quite confused but I figured out the problem after further interrogating of the computer-illiterate who was fiddling with the settings =)
Would suggest a more descriptive error for this case instead of assuming an error with the database setup.