I propose that there ought to be a way for the mods of a site to put up a prominent disclaimer that shows up on every page and makes it clear that the site does not offer professional services.
For example, on mi.yodeya, the SE 1.0 site that was the starting point for Judaism.SE, the header of every page contained the following in bold red:
Like Wikipedia, mi.yodeya makes no guarantee of validity, and does not offer professional (particularly rabbinic) advice. Treat mi.yodeya information like it came from a crowd of your friends.
This disclaimer was the most prominent mechanism used on mi.yodeya to address a concern that many on the site felt very strongly about: that it is inappropriate to turn to a crowd-sourced website instead of to a qualified rabbi for personal, practical advice about implementing Jewish practices, and that a Q&A site would naturally attract people seeking such advice. In fact, this has been a real issue from time to time.
When mi.yodeya migrated into Judaism.SE, we no longer had the ability to insert this disclaimer into our header, even though doing so was a very popular request on our meta almost immediately after migration. Instead, the disclaimer is on the sidebar for new users only and in the FAQ.
It seems to me that it would be more appropriate to bring this sort of disclaimer back in a prominent place for all readers, and further, it seems that there are a number of other existing or proposed SE sites that could use a similar feature:
The new Christianity.SE site is similarly dealing with the problem of people seeking pastoral advice on their site. There has also been discussion about this sort of issue with respect to the proposed Islam site.
It seems to me that Personal Finance and Money ought to make it clear that it's not offering professional financial advice, both to warn people against acting on what they heard from a crowd and to limit its liability.
Even more so, the proposed sites for Medicine and various legal areas could be significantly liable (morally and legally) if they don't make it clear that they're not offering professional advice. In fact, people have already indicated, in the discussion section for a couple of these proposals, the need for such a disclaimer.
I'm not just suggesting this as a pro-forma legal cover against liability, like the email footer Jeff quotes. In fact, I'm not 100% certain of how legal liability applies or could be mitigated. My main concern is that Q&A sites in realms where authoritative advice properly comes from accredited professionals will tend to create situations in which authoritative-sounding advice is offered irresponsibly and acted on improperly. Therefore, askers and readers should be reminded "don't act on everything you read on the Internet," and answerers should be reminded to exercise due humility.
At Judaism.SE, we consider ourselves to be taking on a great responsibility when we say anything about Judaism for public consumption and when we provide a forum for people to do so. To the degree that the potential for would-be professional advice is solicited, offered, or acted upon through our site, we feel that responsibility even more. I get the sense, from the materials linked above, that people in the Christianity.SE, medical and legal communities feel similarly. Given that this type of strong concern is shared by multiple SE communities but is foreign to others, including the programming mothership, I think it makes sense to address it with a feature that can be controlled from within each community.