I definitely want bulbs that are dimmable. Is there anything that'll work, like a CFL, or a reasonably priced LED?
The XPDI3 dimming wall switch will work with
dimmable CFLs. Finding "dimmable" BR30 reflector bulbs may be a problem, but you can use a standard configuration CFL bulb (twisty bulb not in a reflector) IF it says it is dimmable and will physically fit in the fixture. All dimming X10 wall switches now have "Softstart" feature which will damage the CFL unless it is made to be dimmable.
However CFL lights are very noisy, so you should plan on a noise filter, such as XPF
http://www.x10pro.com/pro/catalog/platform.html#filters between the string of lights and the X10 switch. May not be necessary, but be ready.
The XPDI3 is a three wire switch, meaning it requires a connection to the neutral line. So you need a neutral in the wall box.
Two wire X10 wall switches (WS467, WS12A) will
NOT work with CFLs unless there is at least one incandescent bulb in the string (40W or greater). However this is not recommended. When the incandescent bulb burns out it is likely none of the lights will work. Sort of like the series string Christmas lights.