This isn't possible at present, not automatically any way.
However, this will open the form in a iframe, rather than treating it like modal content which is what is done to the login form. The disadvantage to using an iframe is that the form isn't closed upon form submission, like with the login form. The advantage is that the iframe will add a scrollbar to accommodate the size of the content. If you want, you can treat it as modal content (just use rel="lightmodal"), but you'll need to specify the height and width of the lightbox container (see lightbox2 docs).
I'm not adding it to lightbox2 module at this time because there are too many modules that modify the contact form content, e.g. advcontact, contact_forms, content_attach, captcha, mollom, etc. These potentially could modify the size of the content, making it difficult for lightbox2 to estimate the outputted form size.
If enough other people request it, then I'll add the ability for lightbox2 to open the default contact form in a lightbox, but for now, this feature won't be implemented for the reasons outlined above.