The 2.5mm form factor is used for cell headsets usually.

What this might be interesting for is to power/charge a small device (iPod, phone, etc) through a homemade cable. Depends on what the power rating for the outlet is, though - if it's only supplying 10 mA at 3V, it's not going to be too useful. 125mA at 5V, and you're getting somewhere.