I've used Dvorak essentially full-time for the last 20 years and I can still type in QWERTY. In fact, I just switched to QWERTY just now to type this and it feels a bit awkward and I make some typos, but it's perfectly usable.

The hardest part of switching is punctuation and shortcut keys, not the letters themselves. But I can use other people's computers and usually most people don't know that anything's odd.