On the Office Homepage you can find a brief description of the available functions for calculated fields in SharePoint. When I first had a look on this page, I needed the functions for calculated fields, defined via the web interface. In this description you will see, that functions, which will get more then one parameter will separate its parameters by a comma.
When you try to enter this in the web interface, you will get an error, that you have entered an incorrect funktion. When you replace the comma by a semicolon everything works fine.
When you use the Formula-tag in CAML to define a calculated field for a list definition, you have to enter a comma, like in the example. A semicolon will not work in this place. You will also need to write "<" instead of "<", and ">" instead of ">", because it is a xml file.