Laura, howdy! Thanks for your question. I'm not sure of any other literature in regards to creating a walled garden (hence, prob why I pursued it). Are you referring to peer-reviewed research, anecdotal evidence, action research, what exactly?
;->

With appreciation,
Miguel

Laura, have you seen Dr. Helen Barrett's education portfolios work? It's pretty awesome. http://electronicportfolios.org/

I'm not familiar with anything that specific (literature on safe use of digital portfolios in K-12... separate from the whole discussion of safety in Web 2.0 environments) but I sent a request out to the two listservs/groups that I host (on Yahoo and on Google). We'll see if I get an answer.