In general, its not a big deal. However, I think somewhere (maybe a few somewheres) had time off for Christmas and New Years and made some extra spam money. When you consider the number of guests we usually have (0-5 legitimate guests at one time), seeing 300-800 shows there's something up. Now that it's February, things seem to have slowed down a bit.
Really, in a time like that, the only choices are to turn off registration or to insist that any new registers do what RussBrown did and post a small introduction, just so that we can see that you're a real person. However, in my time checking new posts, I've seen three introductory posts, two that were exactly the same and one that followed up with a spam post. I'd really hate to wade through 100 of those at a time but it would be a better indication.
However, I'm sure that any new registers will be happy if we don't do this. Out of about 160 registered people, less than half have ever posted anything. Out of the others, probably one-third to one-half are real people who have signed up at some time or another to get information. The rest are most likely spammers who just haven't bothered to spam.