The multitudes of folks checking my site via Newsgator (...whomever that person is; there are a total of two Newsgator subscribers to Iamacamera, and I'm the other...) haven't recieved a site update since I published my Now Serving application/xhtml+xml article.
Error 500, of course, is a generic server error, and there are sub-codes that can assist me in determining the problem. Based on a Microsoft Knowledgebase entry, I adjusted my log file to include HTTP error subcodes. Lo and behold, W3C and Newgator are generating error 500 with sub-code 0.
Zero, of course, is the generic sub-code error-code, so that eliminates certain possibilities but doesn't point definitively to anything in particular.