First off, the Selenium IDE is rather limited, you should consider switching to Selenium RC, which can be driven by Java or Perl or Ruby or some other languages.
Using just Selenium IDE, you can embed JavaScript expressions to derive command parameters.
You should be able to type a random number into a text field, for example:
type fieldName javascript{Math.floor(Math.random()*11)}
Update: You can define helper functions in a file called "user-extensions.js". See the Selenium Reference.
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