lo123,

I just remembered that sram is making a 10 speed mountain bike cassette:
http://www.cambriabike.com/shopexd.asp?ID=87314

It's expensive, fairly new, I have absolutely no idea idea whether you could mix & match it with shimano - at the very least you probably need to replace the rear derailleur... It's an 11-36 cassette, so it should give you similar gearing.