Can I set iphone to automatically connect to a particular ssid?

There are 6 SSIDs within this office area, the iphone 4 switches between a guest ssid and a preferred ssid.

While connected to the guest ssid, selected "forget connection", within 5 minutes device reconnected to "guest".

Is there a procedure that allows the following?

Configurating any iphone/ipad Device to automatically connect to a preferred ssid when in range, even when other SSIDs are available.