I prefer:

th ()
{
_ if [ -n "${DISPLAY}" ]; then
_ _ # Change X11 kbd layout to QWERTY somehow
_ else
_ _ loadkeys us
_ fi
}

and:

kj ()
{
_ if [ -n "${DISPLAY}" ]; then
_ _ # Change X11 kbd layout to Dvorak somehow
_ else
_ _ loadkeys dvorak
_ fi
}

(where the underscores above compensate for Slashdot's problematic lack of <PRE>). This is something i came up with on my own as well, back in 1996 when i switched. (loadkeys is a Linux-ism, and somewhat old; OpenBSD uses kbd and different layout names, while other BSDs do it yet differently. YMMV).

I found that it took me several weeks to learn (two-handed) Dvorak well enough to touch-type as fast as i could before with a QWERTY layout. Then it took me several more weeks to re-learn QWERTY, which i'm a little slower at now. Thenceforth, i've been able to switch between them by pausing a moment, then deliberately starting to type in the appropriate layout.

For what it's worth....