I always thought 1800s meant the first decade of the 19th century rather than the whole century, so that we went 1800s, 1810s, 1820s, 1830s etc.

I had an architectural history professor who drilled it into our heads that "the 1800's" (yes, with apostrophe) were a decade and not a century. I've given up on converting anyone else to this way of thinking, but it seems "correct" to me. I'm certainly not as adamant about it as she was (and I think she had a British education, Jackie), and I am happy to glean someone's meaning from context. The problem I have is with people who think I'm being too intellectual when I say "nineteenth century" instead. Grr.