This package looks easy to use and I like the use of __toString(). It is also nice to see complete unit tests when the package is still a proposal. Here's some feedback after reviewing the code:
1.) The getHTML() method could use an option to return HTML instead of XHTML.
2.) Return values of all methods could use more documentation
3.) Consider using heredoc syntax for the HTML blob in getHTML()
4.) 'mask_text' is better than 'hide_needle' in Services_ReCaptcha_MailHide
That's all! I look forward to using this package at silverorange.