This is probably why they didn't push WP7 on tablets.
For almost all the OEMs, the average margin on a PC is single digits.
Another key factor to bear in mind is that, at least today, tablets are an additional computing device, and don't replace any existing PC.
http://www.microsoft.com/investor/EarningsAndFinancials/Earn...
I don't think you can say that tablets are bought in addition to PCs, as we've seen PC sales flatline and contract in the U.S. and Europe since the iPad was released. Nobody really knows what the long-term impact will be.
Why? As of iOS5, none of the tablets require a desktop or laptop. I'd venture to say that %90 of people use their home computer for communication (social networking, e-mail, skype) and leisure (games, web-browsing). Why should someone buy something heavy and expensive when a sleek, inexpensive (and dockable) tablet meets all of those needs?
Many full desktop apps will be compilable to run on ARM, and lets not forget MS's own software; full office and internet explorer put windows8 ahead of the competition re: serious work already. (Besides windows phone has already bombed on phones, no reason to suspect it to fair any better on tablets).
Going forward it will be much easier to develop for both platforms. This is a very similar approach to the XBOX; there is an easy way to make software for both platforms.