1 month is common because companies care about their public image when it comes to hiring new people, and they know that the remaining employees will watch how those being cut are treated. If you've got new folks in charge who don't care about it, or need to cut cash outlay _very_ fast because they levered up the company to buy it, you may not get that severance. There's certainly no legal obligation to give a month of severance when it comes to an "at will" employee.
I've been around at a few companies before where cuts happened. The first round of layoffs always got good severance, because the company wanted to make sure that the remaining employees didn't bail... but after 2 or 3 rounds, the jig was up, so the severance shrank from 2-3 months to 2-3 weeks.
There's no minimum severance (at least at the national level), as far as I know. (http://www.dol.gov/dol/topic/wages/severancepay.htm) ... In theory it could be in your contract, but I personally can't recall ever seeing a contract that had such a clause. The standard is "at will" - if you come in to work one day and they don't need you any more, that's it, just the same as you don't really have to give two weeks notice when you leave (but most people do for the sake of the relationship.)
In North America 1 month is pretty common if you've been with a company for more than a year with 2 months not being uncommon.
I've heard of people being given 1 month of notice of a layoff and then being asked to work that month only once:)