Fred,
If you home build it, build less than five, have a ham license, operate on a ham frequency, use it strictly for hobby and recreational purposes, and it does an I.D. very ten minutes or less you are belt and suspenders legal. The five or less homebuilt makes you exempt from the type certification rules. The ham license and ID every ten minutes makes you legal to operate on the ham frequencies (within basic rules of courtesy, non-commercial, and sharing the resource).
With a ham license the DIY transmitters from The Cheap Beep Project are 100% legal if you have less than 5 of them (and no using them for abatement hawks).
As a note on Ron's comments about surface mount parts. The CBP transmitters use what are considered fat and clunky surface mount parts by current standards. These are possible for a first timer to solder by hand. The parts used in current generation consumer electronics can not be hand soldered.
Regards,
Thomas of the Desert