The site, below, has the studies. I think it's pretty recent. This is the summary:

{Studies have consistently demonstrated significant impairment of short-term memory for number sequences or verbal recall, although not for major events or factual elements of narratives. The effects of cannabis are usually subtle, and although statistically significant, the magnitude of any impairment tends to be dose-related and subject to tolerance of the drug"s effects, on the personal characteristics of the user, and on the circumstances in which the drug is taken.

Cannabis may increase the likelihood of minor errors of time perception, verbal recall and similar factors, but not affect recall of important actions observed or experiences undertaken. In a regular user of cannabis, acute intoxication by the drug would have little effect on performance.

Cannabinoids appear to disrupt short-term memory by interfering with the filtering of information, such that a greater volume of information reaches consciousness, overwhelming the ability to store, or prioritise such information for storage in memory. }

http://www.idmu.co.uk/canmem.htm