I was having a discussion with a colleague and we couldn't recall how they both worked. Does 一人以上 mean a) 1 or more persons or b) more than one person?
According to the Wikipedia article on these two, 以下 and 以上 include the number that precedes them.
５０以上 => 50 or more
５０以下 => 50 or less
If you want to exclude the number on the upper end (instead of 以下) use 未満(みまん).
１０００円未満 (less than 1,000 yen, not including 1,000)
I had a few different discussions (and taking Ito-san's point below as well) about this - 超 seems to be used (?) but I've never really heard it. In this case we would probably go back to standard grammar using a verb instead:
If there was really a need to exclude a certain number in the case of 以上, you could always add 1 to it -- e.g.
Although that seems strange too. Maybe there's not much of a use for that kind of term?
As makdad and Axioplase stated in their answers, the “correct” usage is that X以上 and X以下 (where X is a number) include X. However, the “incorrect” usage with X excluded is not rare, and therefore it is better to be aware of this possibility, too. For example, when talking about a ski lift, some people may well say:
このリフトには一人以上乗ることはできません。 This lift cannot carry one or more persons. (What?!)
when they really mean “This lift cannot carry more than one person.”
I think 以上 and 以下 are inclusive above and below. 超 and 未満 are exclusive.