Edit: Just found out that there is a Random() function in the table. This is not in the library, but if you use it anyway, it will tell you that the formula is valid and creates a value from 0 to 1.
The original and still valid answer if you want to use officially supported features:
Since Tableau is used to create graphs based on your data, it is usually not enough to use them for random numbers (could you explain why you need them?)
However, you can use this approach to get around this limitation: http://community.tableau.com/docs/DOC-1474
Basically, getting a semi-random seed out of time, combine it with other values based on tabular calculations and multiplying it by other semi-random values
Seed (DATEPART('second', NOW()) + 1) * (DATEPART('minute', NOW()) + 1) * (DATEPART('hour', NOW()) + 1) * (DATEPART('day', NOW()) + 1) Random Number ((PREVIOUS_VALUE(MIN([Seed])) * 1140671485 + 12820163) % (2^24)) Random Int INT([Random Number] / (2^24) * [Random Upper Limit]) + 1
Where [Random upper limit] is a user-defined value to limit the range of results.