A team of hardware modders has created a DS cartridge
that allows the system to interface with devices using Bluetooth. Since it's a Slot-1 cartridge (and thus fills the space normally occupied by DS games), the DS Bluetooth cartridge isn't going to be of much use in games, homebrew or otherwise, but it does
allow the system to "scan its neighbourhood and establish connections to other Bluetooth devices."
Using the API provided by the team, homebrew users should be able to write custom software to interface with any device that uses Bluetooth, from phones to GPS to ... PS3 controllers? The device sells for €59, but the open-source-friendly team released full schematics so you can build your own.