Miss Manners, maybe
Yvonne asked an etiquette question and it's a good one, and I think the answer would be highly personal depending on the blogger. See, it goes without saying we bloggers like attention, are shameless navel-gazers, and want to be read and adored and we want to be talked back to. At least that's the theory and every blogger is different.
The cool thing about blogging is that we can talk to the world at large without any real fear of consequences. I don't actually ever expect to run into someone who reads this blog regularly (unless you're my mother, in which case, HI!) and I would be honestly weirded out if someone found my blog and then met me and associated the two of us together since I try to keep at least some modicum of privacy. If this happened to me, I'd probably think the person was a stalker, especially if I had not told that person I had a blog (and generally, I don't tell people about the blog except some close RL friends).
So my advice is, if you think you've met a blogger in RL and even if in a non-stalkerish way and also in a way that is completely unrelated to the blog, I still wouldn't say anything at all unless the person ventures out information about the blog. In fact, until you get to know the person better, I'd say nothing and maybe down the line, if all is well and good, then you can admit the truth if you want to. (And of course, all of this is moot if the person posts their full name, picture, etc., on their blog and at that point, I think it's fair game to say you read their blog, because really, there's nothing stalkerish about that and the blogger is being quite obvious about wanting to be found out). But in general, I wouldn't say anything, because strangely, despite all the soap boxing, blogging is kind of a private thing.
Any other bloggers have thoughts?