Which version of FM?
rdowler asks a good question.
If you have Filemaker 10 or 11, you can use the OnObjectModify script trigger to run a script that uses Show Custom Dialog with an input field to capture the "other" text.
If [ Not IsEmpty ( FilterValues ( YourTable::Yourcheckboxfield ; "Other" ) ) ]
Show Custom Dialog ["Enter other value:" ]
Excellent! FMP 11 it is, so I'll give that a crack. Thanks for your help.