In regards to OS support, the only OS I have really had problems with is Windows. Maybe I just wasn't checking the right checkboxes, but Windows seems to default to "I assume you want a different keyboard layout for each window, so have fun with that!" OSX and Linux have always been very sane, defaults-wise.
Changing layout on OS X and Gnome (I'm not sure about Win) is quite fast, usually if I need to type on someone else computer for more than 10 minutes I tell them and change it.
Also even if I've switched to Dvorak a year ago I can still type at a good speed on qwerty.
I've been using Dvorak for 12 years. Initially my QWERTY speed was seriously hampered, but after being in environments where switching was less convenient (public labs and the like) it recovered.