We exploit the properties of ultranarrow, Fano-like resonances generated by the coupling of long range surface plasmons with ultrathin (~10 nm), metallic, subwavelength gratings embedded in a nonlinear, cubic material to obtain all-optical switching at telecommunication wavelengths for extremely low input power. We provide an example of a silver metallic grating embedded in a chalcogenide glass (As2S3), and we show the concrete possibility to achieve all-optical switching at local field intensities compatible with the photo-darkening threshold of the material.