What special requirements are there for making game art that supports High Definition? Does it require big changes in the way you model or texture your game objects? For instance, do models need really high resolution textures to support HD? If so, how high? And do you have to have a particular game engine that supports HD or would any game engine do? (I'm using Unity - would Unity work?). Also, are PS3 and XBox 360 the only platforms for HD or could PC and Mac handle HD games? Thanks.
Saying your game supports (or is) HD, basically means that it can be output (/displayed/rendered) in an HD resolution (for television screens). For a 1080p TV the display resolution is typically 1920x1080 pixels and for 720p it's 1280x720 pixels, assuming a widescreen aspect ratio of 16:9.
Most modern computer monitors are able to display these resolutions and others crisply without a problem. However, older TV input standards (i.e. "standard-defition") required that the display resolution was much smaller and so older video game systems such as the N64 and PS1 would effectively be outputting to resolutions of 640×480 (NTSC, 480i). There's also the issue of 1080i vs 1080p but that's outside of the scope of this question.
If a game says it's HD, it means it supports, and was designed for, a high definition resolution. So when you run the game at 1080p, the graphics will be crisp and clear, and the framerate will be consistent1.
The fact that you have so many more pixels than if you were rendering to, say, 640x480, means that if your textures are low resolution, you'll be able to tell, or more importantly, your players will be able to tell. As for models, if your models are square and blocky, then yes, this will be noticeable and you'll also want to make new versions that look round and smooth in higher resolutions. Perhaps this can be solved with textures, but that's for you to decide when you're designing it.
I think that's something you'll have to figure out once you run it in an HD resolution. The better the quality of the textures, the better things will look in game (theoretically). But they also take up more memory and may take longer to load. Maybe you'd want to prioritize what textures have the best resolution (i.e. the main character that the player is going to see ALL the time) vs ones that may not necessarily be important (i.e. the top of a mountain).
I think most game engines should support changing to/defining a new resolution. But they may not be able to render at certain resolutions. For example, an engine may support 720p but not 1080p. This does, of course, depend mostly on the game content. Any engine can render a black screen at 60fps at 1080p. :-)
PS3 and Xbox 360 support HD resolutions up to 1080p (depending on model for Xbox; also, despite the max resolution, I believe most HD 360 games are 720p). The PC/Mac depends on the model and monitor so there's not really a firm "yes/no" in that category.
1) Consistent Framerate - At least on console, since you know the hardware limitations. It's difficult to ensure the framerate won't fluctuate on PC since you can't control the player's hardware.
HD isn't really a PC term at all. It's a TV term. All it means is that the TV has a high resolution. Nothing else. So if you make a game for PC, then an HD TV would show it just as crisp as it is on the PC. Although HD is just 1080, so if your PC screen has more, then not really... I mean, my laptop has more...