Just want to mention, as an update, this is now trivial with Splunk 6.1 by using the multiselect input type.
<input type="multiselect" token="my_animals">
<label>Types of animals in my soup</label>
<populatingSearch earliest="$earliest$" latest="$latest$" fieldForLabel="animal_name" fieldForValue="genus_species">
index=fauna | stats dc(genus_species) by animal_name | fields animal_name genus_species
</populatingSearch>
<default>monkeys,lions</default>
<delimiter>OR</delimiter>
<valuePrefix>(my_animal=</valuePrefix>
<valueSuffix>)</valueSuffix>
</input>
Now you can use the token $my_animals$ in a search and it will expanded into (my_animal=lions) OR (my_animal=monkeys) OR (my_animal=tigers) OR (my_animal=bears)
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