Turn on SQL error display (see DefaultSettings.php for all configurable settings).
Excellent - thanks, Brion - Found it. Added a line to the bottom of LocalSettings.php...
$wgShowSQLErrors = true;
...and now get the real error...
===--- A database query syntax error has occurred. This may indicate a bug in the software. The last attempted database query was:
SELECT cur_id,cur_namespace,cur_title FROM `cur`,`links` WHERE cur_id=l_to AND l_from=1081 FOR UPDATE
from within function "LinkCache::preFill". MySQL returned error "1064: You have an error in your SQL syntax near 'FOR UPDATE' at line 3 (localhost)". ===---
1081 is the id for the page I was editing in the 'cur' table - I found that last night. After creating a link from page ID 5 to page ID 1081, there wasn't an entry in the 'links' table with l_to == '1081', which I guessed there should be. I tried adding one manually, but that didn't work, and I figured playing with the SQL data directly like that might not be the smartest thing to be doing - especially at 02:00 ;)
Thanks,