OK, I went ahead and truncated wp_1_sferrorlog after exporting the most recent 100 errors. Funny thing is, the last errors appear to have been thrown back in October. (Or, I didn’t actually dumb the latest records.) 
Here’s a couple example errors that repeat in the export:
(72187841, ‘2012-10-16 13:48:16’, ‘php’, ‘file: /simple-press/forum/content/classes/sp-group-view-class.php<br />line: 268<br />function: sp_groupview_query<br />Warning | Creating default object from empty value’),
(72187842, ‘2012-10-16 13:48:16’, ‘php’, ‘file: /simple-press/forum/content/classes/sp-group-view-class.php<br />line: 314<br />function: sp_groupview_query<br />Warning | Creating default object from empty value’),
Anyway, it required restarting both HTTP and MySQL, but the site seems to be back up now and the forums are functioning as expected. I’m going to presume the massive error log was bogging down the database.
A couple concerns at this point…
1. Doesn’t the error log empty itself automatically or only hold x number of records? Or should we be manually emptying the log periodically?
2. It looks like the log is already filling up again with the following errors:
file: /simple-press/forum/content/classes/sp-member-view-class.php
line: 348
function: sp_memberslist_query
Warning | Creating default object from empty value
And after emptying the log (from sp admin) and reloading a couple pages:
file: /simple-press/forum/content/classes/sp-forum-view-class.php
line: 451
function: sp_forumview_query
Warning | Creating default object from empty value
3. How can I ensure this doesn’t happen again?