I've heard several times that you'd have to use a ceramic mug "a thousand times" before breaking even from an energy point of view, but I've never been able to document that.
But I did find this interesting document: "Report of the Starbucks Coffee Company/Alliance for Environmental Innovation Joint Task Force." What I really liked about the report is that they looked at lifecycle costs. Here is a quote from page 10:
The Alliance conducted an environmental analysis of the full life cycle of ceramic, paper,glass,and polyethylene terephthalate (PET) plastic cups,from the extraction of raw materials to their manufacture, use, and disposal.The Alliance found that the breakeven point beyond which environmental benefits began to accrue was approximately 70 uses for ceramics and 36 uses for glass. Given that a reusable cup may be used, on average, 1,000 times or more (and is generally designed for 3,000 uses), the environmental benefits of using reusable cups in terms of reduced energy use,air and water pollution,and solid waste can be tremendous.
Also take a look at the Reusables Analysis on page 12 where the authors look at costs for a typical coffee shop, including annual water savings, greenhouse gas reduction, and solid waste reduction.
Now instead of cups, the conversation has shifted to the break-even point for hybrid cards and photovoltaic cells.