I could do with a new rear mech, currently running 9 speed x0/x9 mix. I have about 4 or 5 months use left in my cassette.

Are 10 speed SRAM shifter compatible with 9 speed medium cage mechs?
For that matter are 10 speed shimano shifter compatible with 9 speed shimano mechs?

Plan to go 1×10 with an 11-36 cassette when i need to replace my drivetrain. Don't fancy buying a new mech if it's only going to be compatible for a few months.

The rear mech is designed to move across the width of the cassette, being pulled by the amount the shifter pulls it. 9 and 10 speed cassettes are the same width. So long as you don't mix the SRAM 'X' shifters with shimano mechs it should be fine.

Someone else will be along in a minute to tel us that's tosh though.

Are 10 speed SRAM shifter compatible with 9 speed medium cage mechs?
For that matter are 10 speed shimano shifter compatible with 9 speed shimano mechs

NO !

For that matter are 10 speed shimano shifter compatible with 9 speed shimano mechs

NO !

Explain why not?

I've also been thinking about switching to 10 speed, and I figured all I would need is new cassette chain chainrings and shifters.

Maybe 10 speed jockey wheels? Would they fit in a 9 speed mech?

its been done loads, use the search

2011 products are not backwards compatible due to a change in cable pull

As dirtyrider says, new cable pull on 10 speed stuff (SRAM or Shimano), so it's not reverse compatible.