With the past few decades of generally warmer weather in the US, homeowners, gardeners, and landscapers face a quandary in deciding what plants are suited to the local climate and when to plant them.
Gambling with treacherous weather has always been part of the deal, of course. But in the past, planters and growers have received general guidance from the US Department of Agriculture, which for more than four decades has made available a map of plant hardiness zones. The zones reflect the average annual low temperature, offering a good general starting point for deciding how much cold tolerance a plant needs to survive, but don't address local microclimates influenced by wind, shade, terrain, and other variables. USDA updated its map in 1965 and 1990 as better and more recent climate data became available. USDA Agricultural Research Service: Kim Kaplan,  301-504-1637. 1990 Map  (based on 1974-1986 data).
Another update had been in the works, contracted out to the American Horticultural Society (and its subcontractor, Meteorological Evaluation Services, Mark Kramer,  631-691-3395). Using data from 1986 to 2002, the Society worked with the agency, developed a map, and published a draft version in the May-June 2003 issue  of its publication, The American Gardener (David Ellis,  800-777-7931 x122).
The Society anticipated USDA would soon issue its final approval and publish the map itself. However, the agency concluded 3-4 months later that the map was flawed and decided not to finalize it. One reason USDA cited was that the revised map, which shows that the vast majority of areas aren't getting quite as cold, was not based on a long enough time period. The delayed map was based on 16 years of data (a bit more than the 13 years used in USDA's 1990 map), but USDA officials say that a longer interval, say 30 years, better reflects the typical year-to-year vacillations in the weather. Critics say the effort to use older data only serves to mask the effects of the steady rise in temperature over the past 20 years or so, and accuse the Bush administration of squelching the release of the updated map for reasons of climate-change politics.
Another reason for rejecting the draft map was that it frightened a cadre of nursery operators who had the ear of the USDA. The growers were concerned that some people would try out borderline plants that could die when the occasional cold wave still hit, which would lead to angry customers.
That concern has some merit, says H. Marc Cathey, who directed the development of the 1990 USDA map during his 37 years at the agency. He also is president emeritus of the Society, and coordinator of the Society's map (704-896-1209). With those multiple perspectives, he supports the use of the 16-year interval of the Society's map.
USDA is now working in-house on an update, with oversight from an advisory committee made up of USDA staff and outside experts (one member: Tony Avent, Plant Delights Nursery, 919-772-4794). The update is expected to include data for the years 1974 through about 2001-2003, and the hardiness zones may be only slightly different from the 1990 version, Avent says.
The map is expected to be an interactive Web version, and may be more finely tuned than the AHS version, due to the use of more complex calculations folding in factors such as wind and elevation. However, three years after the release of the initial Society's map, there is no public timetable for release of the USDA update.
Meanwhile, the National Arbor Day Foundation has developed its own update,  based on 1987-2001 data from the US National Climatic Data Center (Robert Smith, 888-448-7337 x256). That map, which differs slightly from the Society's map, shows a widespread shift of the average low temperature to the north or inland in almost all areas of the country.
Such shifts are beneficial to plants to some degree, since the likelihood of cold killing them is reduced. On the other hand, the potential for excess heat also increases, posing its own set of threats.
To help address the issue of added heat stress, Cathey and the Society developed a Heat Zone Map  in 1997, based on 1974-1995 data. This map may be updated in a few years.
Despite the limitations of generalized hardiness maps, Cathey suggests using either the Society's or the foundation's maps as a starting point, until USDA unveils its update, a development he is closely following in his retirement. Even then, those inclined to gamble that the warming trend of the past 15-20 years will continue may want to stick with the other two maps.
Another challenge at this time of year is anticipating when it's safe to put out garden plants that can't survive a frost. Most areas have a widely circulated average date of last frost. But that too is shifting, according to Kramer, the Society's consultant, who likely can provide you with more details. In addition, to help people better roll the dice, you may want to direct them to the National Oceanic and Atmospheric Administration's 30-day forecasts. 
Sources such as local nurseries, landscape architects, and county extension agents likely can add a range of opinions.