Read this: http://www.sqlskills.com/blogs/paul/a-sql-server-dba-myth-a-day-1230-tempdb-should-always-have-one-data-file-per-processor-core/

In general, here's what I'd say. Separate LUN if possible, but not critical. Go with # cores for <= 8. After that, stick with 8 unless you can test.