in org-mode, is there a function to export (current) subtree to HTML?

With Org 8:

M-x org-export-dispatch
C-s (change the export scope to subtree)
h (HTML menu)
h (export to HTML file)

You can also use org-narrow-to-subtree to reduce the buffer and run the normal export.

When transient-mark-mode is turned on, only the active region will export.

The information for ODT export found here, also works with HTML.