I believe it depends on your usage. For a lot of creative professionals I can understand the justification for 30inch displays. Two or four even. If your doing photo / video / sound post production all day for a job, they probably pay for themselves the time they save.
For the majority of people their computers aren't used for heavy creative work, mainly typing, email, web surfing, IM, calender etc. For these types of work I am from the school where 13-20 inch screen sizes are ideal.
I am a photographer who is happy working on a 15inch screen but I could see how plugging into a 23 or 30 inch display would improve my productivity - that doesn't help me when I am on location though.
I find that the study about 23 / 24 inch monitors being more productive than a 30 very interesting. I have always had a preference for this size. The 24inch iMac is used by a lot of my colleagues and I have to say, it is wonder to see photographs that large.
As for a 30 inch display, like I said above - probably far to big for 95% of computer users, but then we are Mac users, and if we work in a creative profession it is probably justified that the larger the better!