I know that the American Chemical Society publishes the ACS style Guide which includes a chapter on references. Their general rule for reference citing is:
For material other than books and journals, sufﬁcient information must
be provided so that the source can be identiﬁed and located.
They identify several electronic sources outside of electronic versions of books and journals that include: online encyclopedias; web sites; and electronic lists/newsgroups.
If we consider Stack Exchange Network a "General Webs Site" then the author would likely not be the users who answered the question, but rather the authors of the service provider, most likely Joel Spolsky.
If we consider Stack Exchange Network an electronic list or newsgroup, then no author is provided, for example:
Computational Chemistry List, solvent discussion in archived messages of September 2003, [email protected] (accessed Nov 10, 2004).
In skimming through the document, those are the only two formats I could find that would meet the standards of a suitable reference while avoiding the tricky issue of using avatar names.
While we're at it, the use of pseudonyms suggests some level of anonymity, which may place StackExchange Q&A into a separate category:
Whether the message was personal and set only to you or whether it was posted in a newsgroup, it is not published. E-mail messages should be cited the same as any other personal communication.
So if there is a case where we need to consider StackExchange Q&A as personal communications, there are ethical considerations for citing such information:
Information obtained privately, as
in conversation, correspondence, or discussion with third parties, should not
be used or reported in the author’s work without explicit permission from the
investigator with whom the information originated.
I suspect this last point is not an issue, since all users of the Stack Exchange Network have agreed that material posted here falls under Creative Commons.
In your case, it might be appropriate to insert a reference to the Stack Exchange Network in the same section you use to cite the software used for data acquisition and manipulation. Then, in the acknowledgements section a general thank you to the "active users" of M.SE. It may be nice to provide an opportunity for those who have helped to provide their real names, as you never know if @belisarius is actually a 12 year old savant looking for early admission into Harvard and needs some more name recognition.