Answer: Not many, at least when compared to IT.
The DOL just has published its Factsheet: H-1B Temporary Visa Program - Selected Statistics, FY 2012 Year-To-Date and the paucity of Healthcare petitions is startling. The IT community uses the vast majority of H-1Bs. Eight of the Top 10 occupations are IT jobs. All ten of the largest H-1B employers are IT -related companies. So how many H-1Bs are used by Physical Therapists, Occupational Therapists and other healthcare occupations? Unfortunately the DOL does not expand beyond their Top 10 lists.
The Permanent Residency data is a bit more forthcoming. At six percent of the overall total, "Healthcare Practitioners & Technical" was the fifth greatest occupational classification in terms of approved Labor Certifications. Six percent was 2,100 approved Labor Certifications and includes Registered Nurses, Physical Therapists, Occupational Therapists and about one hundred other jobs. These numbers still pale in comparison to "Computer/Math," which makes up 54% (18,800) of all approved Labor Certifications.