I found a really good price on a step dimming
http://cgi.ebay.com/GE-B132R277S50-1-LAMP-DIMMING-ELECTRONIC-START-BALLAST-/350359629215?cmd=ViewItem&pt=BI_Security_Fire_Protection&hash=item51930fad9f
this ballast uses 2 switches if both are on, then 100% lamp output, 50% output for one switch
it has NO EOL protection, which could be interesting
see this also
http://www.unvlt.com/productLines/ap_sheets/ELFB/B132R277S50.pdf
sorry about the URL! i never learned "the right way to do it"
It's an older dedicated voltage ballast. 1 x 32W, 277v input.
It looks like step dimming ballasts are using two switches now a days. Advance used to make something interesting. It was a four lamp ballast with 4 lamp on startup, then rapid power cycling makes it switch to 2 lamp.
http://www.brite-lite.com/Pdf/Form1939.pdf