Well, there are many people interested in this (and I would be, too). Unfortunately, it is not yet possible because these browsers lack support for a few basic things required for such an application to work. Examples are:

• document.execCommand and document.designMode interfaces ("lovingly" known as MSHTML or something like this)
• support for DOM2 Traversal/Range interfaces. IE doesn't have it, but has something similar (well, totally incompatible with the standard, but at least it can be used: "TextRange")
• a keyboard interface or contextual menus is almost impossible to create in these browsers
• Safari seems to have a high number of bugs that cause a browser crash. JavaScript debugging is usually impossible.
• both of these browsers are extremely slow! I can't imagine how Opera folks can claim it's "the fastest browser on Earth"!  Indeed, Opera renders pages really fast, but DHTML stuff is much slower than in Firefox.  Same goes for Safari.
  

Currently, in my opinion, the only browsers providing useable DHTML support are Firefox (and any Gecko derivative) and IE 6/Win. [but to make things clear: this opinion relates to really complex DHTML applications; Opera/Safari may be very good for day-to-day Web surfing, but dynamic stuff just won't work right; both of them are years behind Firefox or even IE at this point].