Over at MarsDrive Consortium is a fascinating document entitled MarsDrive Reference Mission: "Mars for Less" (1.9 MB PDF). It is a nicely explained and illustrated description of how a Mars Direct type manned Mars mission could be accomplished through the use of multiple launches with medium lift launch vehicles sooner and more cost-effectively than with heavy-lift launch vehicles.
While HLLV's are certainly desirable, their development time and costs are very high, meaning that they will probably not be available for many years, assuming their development survives the budget hurdles of multiple administrations and crises. There certainly are also challenges in assembling large Mars-bound systems in Earth orbit, but this paper explains that these are not insurmountable, and in some respects, on-orbit assembly represents the most well-tested aspect of manned Mars missions. And there are already MLLV's that can do the job, with more in development, so development delays and other problems with one launch vehicle do not necessarily lie in the critical path of the entire mission.
This paper was written by Grant Bonin based in part on earlier articles he published in the Journal of the British Interplanetary Society, Vol. 58, 2005, and The Space Review, January 2006.