To give a U.S.-based reply: :-D
No pre-Intel Mac will be able to handle direct decoding of the new "ATSC" digital TV signal. (Well, in theory a fast G4 or single G5 could handle the 480p broadcasts, but most 'primary' channels are 720p or 1080i, which EyeTV says a Dual G5 or Intel Core Duo is needed for. And, no, you can't just try to decode at 480p, the machine has to be fast enough to decode the native 1080i signal, no matter what resolution you're trying to display at.) So 'native' support is nonexistent for older machines. Not to mention that all ATSC-compatible tuning software requires at minimum 10.3. (And the latest version of EyeTV needs 10.4.11 or later.)
As others have said, you can use a converter box, though. These are the devices that the U.S. government was subsidizing last year, that, with the government coupon, you could get as cheap as $10. These decode the new ATSC signal, and output standard old fashioned (aka: "low-def") NTSC, either over coax (tune a TV tuner to channel 3,) or over composite (yellow-red-white cables.) From there, you can use any TV-input source to watch; you just use the converter box's remote control to change channels. (Yes, you can even watch on an old Macintosh TV this way. Or any other old Mac with composite inputs, like a beige G3 with "Wings" AV input card.)
No pre-Intel Mac will be able to handle direct decoding of the new "ATSC" digital TV signal. (Well, in theory a fast G4 or single G5 could handle the 480p broadcasts, but most 'primary' channels are 720p or 1080i, which EyeTV says a Dual G5 or Intel Core Duo is needed for. And, no, you can't just try to decode at 480p, the machine has to be fast enough to decode the native 1080i signal, no matter what resolution you're trying to display at.) So 'native' support is nonexistent for older machines. Not to mention that all ATSC-compatible tuning software requires at minimum 10.3. (And the latest version of EyeTV needs 10.4.11 or later.)
As others have said, you can use a converter box, though. These are the devices that the U.S. government was subsidizing last year, that, with the government coupon, you could get as cheap as $10. These decode the new ATSC signal, and output standard old fashioned (aka: "low-def") NTSC, either over coax (tune a TV tuner to channel 3,) or over composite (yellow-red-white cables.) From there, you can use any TV-input source to watch; you just use the converter box's remote control to change channels. (Yes, you can even watch on an old Macintosh TV this way. Or any other old Mac with composite inputs, like a beige G3 with "Wings" AV input card.)