There are a lot of people especially with diseases such as Multiple Schlerosis who have found canabis very helpful medicinally. Really for pain control rather than actually curing.
The problem has been that due to the recreational history of cannabis it's been hard for drugs companies to get licenses to investigate it and they've been slow to get involved for other reasons.
More recently though a number of them have been looking at deriving useful medical drugs from cannabis. You can't just prescribe 'a joint' because a) there are a lot of toxic compounds in there too b) you have no idea exactly how strong a particular batch is c) people would sue you when they got lung cancer :c)
There's a page on current work here, they say possibly available in 3 years probably as a nasal spray
http://www.idmu.co.uk/gw060400.htm