Using /entitydata to apply the NoGravity:1b tag to the entity produced by the fishing rod when used (named unknown by default) succeeds but has no effect on the entity.

To reproduce:
repeat cmd block: entitydata @e[name=unknown]

chain cmd block: execute @e[name=unknown] ~ ~ ~ /particle dragonbreath ~ ~ ~ 0 0 0 0.001 5

When inspected with empty entitydata command the bobber clearly has the tag, but it still flies in an arc as marked by the particles.
Note: it doesn't crash as stated in another bugreport, no idea why.