Blu-ray
Columbia
Disney
Fox
MGM
Lion's Gate
Warner
New Line
Weinstein
HD DVD
Universal
Paramount
Dreamworks (live-action)
Dreamworks Animation
Warner
New Line
Weinstein
A Sony insider on blu-ray.com has said Weinstein titles are in the works for Blu-ray, but no official announcement has been made. Also, Dreamworks and Dreamworks Animation are COMPLETELY SEPARATE companies. Dreamworks is owned by Viacom and is a sister company to Paramount (though they operate independently and publish on DVD under the "Dreamworks Home Entertainment" label, which Paramount distributes, just like Warner distributes for New Line who makes decisions independent of Warner). Dreamworks Animation is it's own company, but has a distribution deal with Paramount.
Confusing, but an important distinction.
That said, since Universal has been the most aggressive studio in number of releases, the support is a lot more balanced than it seems despite HD DVD not having Sony, Fox, and Disney.