I'm sorry. I know it'll create a mild annoyance. I had thought about doing it for quite some time, because I'm sick of dealing with foreign-language spam ads masquerading as comments.
Some of you might be wondering what I'm talking about because you've never seen said comments. That's because I clean them up completely.
The other option here is comment moderation, but IMHO the CAPTCHA is preferable because (a) I don't have to deal with Japanese porn, or whatever the heck is being advertised, and (b) comments will show up when they are made, not later.