As moo says, MathType has LaTeX input support, and export.
With some jumping through hoops you can get it with OpenOffice and rather easier with AbiWord.
But I'm not an expert on modern versions of Word, our last Machine with Office is currently at the PC doctor (HD crash during a lightning strike on local Electricity substation), and when it comes back it will have a clean install of XP (plus new graphics card for son's games), and I will not be reinstalling Office.
For portability I am inclined to write TeX using a text editor when away from any of our three PC's with LaTeX.
It isn't that, I know LaTeX at least proficiently, it is because lets say I am typing this math document that I currently am. Its a little late to switch mediums since it is like 125 pages, but all these commands that I know in LaTeX and want to use in word, it doesn't know the commands. I mean it knows a lot of them \prec, \infty, \int_a^b etc. But some it just doesn't.
I spent a few years with MathType on Word, and quite liked it. I was very taken with the UI which was sweet. Problems, though:
a) Word documents became unmanageably huge with only a modest number of equation labels/links in.
b) Linking equations between different word documents was not easy.
c) Managing an extended project consisting of many word documents was unwieldy.
d) It made the computer crash.
e) I could not get MathType to work with OpenOffice without causing OO to crash.
When I upgraded my computer and found I had to buy a new copy of Word to go with it, I cut my losses and learned LaTeX and for the last two and a half years have been continuing working on the project I had started as a set of Word documents now completely in LaTeX. The project is now well over 1000 pages long and is *still* manageable.
For my next trick, I'm working on attempting to convert it into a website. This is not a trivial task ...