postgresql wildcard when parameter is -1

Hi guys,
I'm still running into problems on my front end, but thats because of access, not postgres ;)

I have a question, when a parameter is recieved as -1 (as an integer from Access), I would like that parameter to be a wildcard. The logic is that I am sending the primary key for a table to postgres for a report but when there hasnt been a value chosen in the combo box -1 is sent to postgres so it would mean that that parameter don't apply in the SP, otherwise if it did have another value then the parameter would be used to do some filtering (joins).

Any ideas? Feel free to ask any questions if you want.

Cheers,
Ben