His only mistake was sequencing the BBC interview before the full speech. The former was a highly condensed version of the latter, which then got condensed even more in the summarising and became the narrative through which the subsequent uproar was mediated.
The line in the interview about aspects of Sharia being "inevitable" made sense in the broader context of the speech but was a gift to the haters. Responsible people are of course reading the transcript of the speech but it's even better to listen to it (54 Mb MP3 file, about 55 minutes long), because that captures the understated and cautious tone of what he was saying -- as does the lack of reaction from the crowd, many of whom must be surprised at how what they heard had turned into such a caricature within 24 hours.
The over-the-top reaction from the American right is something to keep in mind for the next controversy about a pharmacist who won't sell birth control pills or the next time that George Bush says he needs an exemption from federal law to allow some faith-based initiative to receive government funding.
A less serious Rowan Williams post