The last IPCC report on the physical science of climate change, called the Fourth Assessment Report (AR4), was published in 2007. Since that time, plans for the next assessment (AR5) have been underway.
I think I posted before about the announcement of the chapter outlines and the lead authors [link]. The groundwork has been laid for some time, with the lineup of authors finalized earlier this year [pdf]. A lot of the effort falls upon Thomas Stocker [link], the Co-Chair of "Working Group 1," the job previously held by Susan Solomon [link] for AR4. Not only that, but there have already been two meetings of the lead authors, the most recent being in July in Brest, France.
If they are able to keep on schedule, then the AR5 will be published in two years: in September 2013.
The first order draft will be done and reviewed in the next few months. As you know, the assessment is basically a gigantic review paper of climate science. As such, it relies on published results for the basis of the review. The last day to have papers submitted to journals and be considered for inclusion in the AR5 is 31 July 2012, so if you want to get something into the next IPCC report, you have about 10 months to get it done and submitted. It also needs to be accepted by 15 March 2013, giving you plenty of time to deal with those pesky reviewers.