i've been wondering about ham callsign plates for motorcycles myself. the ARRL site (http://www.arrl.org/amateur-license-plate-information) only seems to shows explicit answers for CT (yes), TN (yes), and WY (no). though the WA form (http://www.dol.wa.gov/forms/420499.pdf) linked to from there seems to say it's possible. sadly, apparently not yet available in MA.