Health Care, Health-care, or Healthcare?

Merriam-Webster's Collegiate Dictionary (2003) says that health care is two words when used as a noun.  It is hyphenated when used as an adjective (as in health-care providers). 

In U.S. English, hyphenating a multiple-word adjective is standard practice unless the dictionary rules otherwise. 

However, many U.S. writers, including corporate writers, have started using healthcare as one word.  For example, one of my early clients was the Metropolitan Chicago Healthcare Council.  Another (now functioning under another name) was called Healthcare Compare. 

U.S. dictionaries are compiled by lexicographers who survey how we write and speak in both public and private discourse.  As a result,  in the next edition (2013), Merriam-Webster's Collegiate Dictionary may well list healthcare.  To write the adjective healthcare now is, perhaps, to be ahead of your time. To write the phrase health-care providers is safe.