Form validation: how to use is_unique(table.field1 + table.field2) ?

[eluser]Genki1[/eluser]
Question:
When validating a form, can the IS_UNIQUE() rule validate that a combination of two fields is unique in the table?

Scenario:
Each user has a list of words. I want to know if the word to be inserted is unique to that user.

Table: users_words
id
word
user_id

Example:
Is the word "happy" unique to user ID #1?

Of course, I can write this simple validation routine, but it'd be nice if the functionality is already built-in.