Pretty sure it'll be that "soft touch" coating that some manufacturers have been using lately. Feels a bit like a smooth rubber but if you try to dig your fingernail in it feels pretty much as hard as plastic. My Logitech MX5500 combo has it on the palmrest and non-thumb side of the mouse. It can wear off though, and my home keyboard palm rest has gone all tacky
I'm not sure of the technical name for it or what it is made from, but I'm going to guess some sort of polyurethane? I originally thought it was a surface treatment (i.e. modified the surface, not adding to it) but after a few years it seems quite clear its an additional layer of a different type of material.
Could be different though as the ones I have with the coating are grippier not slipperier? (although newness can affect it). Theres probably different tackiness formulations of it though. (Some phones also have it as the backing, I think the Motorola V3G (the fatter 3G one whatever it was called) had it).
Edit: This kinda stuff maybe?
http://www.bayercoatings.de/BMS/DB-RSC/BMS_RSC_CAS.nsf/id/COEN_1A5Softfeel(Although, I double a Chinese IR device uses genuine Bayer chemicals
)