[Tamlin]

You could either create your own effect by following one of these tutorials:

Rusty Metal Texture in Photoshop - Outcast Studios
Create Rusty Metal Text in Photoshop: Tutorial | Portent Interactive, Seattle, WA
Dirty, Pitted Metal
OriginMaker Create a Rusted Metal Sign in Photoshop
Fiery Darts Creating a Rusty Surface Texture Your Source for Unique Adobe Photoshop Tutorials
Photoshop Rust Tuotorial
Photoshop Rusted type Tuotorial

or simply apply a Layer Style like one of these:

BioRUST.com :: Downloads >> Rust
BioRUST.com :: Downloads >> Industrial
Adobe - Photoshop Styles