I have a shetland which I plan on riding - he's 41 inches and I'm 5'2 and just over 8 stone. He's not broken in yet though and I think I may be a little heavy to break him in but once he's matured a bit and has got a bit of muscle I think he'll be ok.
There's a fab forum just for shetlands, there are some fab pics of adults riding ponies on there, someone even took their hunting!
I'll try and find the link for you
This has opened me up to a whole new shettie world