Okay, status update on Wikithulhu: I've got the Selected Letters of Clark Ashton Smith, Selected Letters of H. P. Lovecraft, Clark Ashton Smith: Letters to H. P. Lovecraft, and Hail, Klarkash-Ton! all done - that is, abstracts of all letters, each page tagged, and arranged in order of appearance and date (a little rough on some of the dates, but hey), and listed on the Index. So that should cover the bulk of the published HPL/CAS correspondence (though there's a couple odd letters floating around in various places, I'll get those as I get time). For anyone that's interested:

[www.wikithulhu.com]
[www.wikithulhu.com]
[www.wikithulhu.com]
[www.wikithulhu.com]

And if you just want the Index, it's on a spreadsheet:

[docs.google.com]