There are very few folks out there who will speak negatively of the quality of Harris' bipods. They are sturdy, versitile, and competitively priced. Most shooters will never wear one out.
Now, there are other options (like Parker Hale) which cost a lot more but are also very high quality pieces (they seem to be built like a truck)
It seems that you may be looking for something with a high "cool factor", while not sacrificing quality. (You're a person after my own heart.)
I like the looks of the M-5 bipod offered by DPMS, but I haven't actually seen one in person (maybe someone can ring in on its quality, durability, etc?) It has a unique look to it, but it's not as flexible as the Harris 'pods. (Non extendable legs, the legs seem to be made to look cool, but there isn't much function in their looks.) Maybe this is one for you...
[url]http://www.shooterstore.com/acb/showdetl.cfm?&DID=90&Product_ID=6438&CATID=494[/url]
Re. Harris' AR "adaptor": It consists of a steel plate that fits inside an A2 handguard (not sure if it would fit an M4's double heatshields). It's tapped (threaded) to accept a threaded sling swivel stud and bracket from the outside of the handguard. The standard bipod actually "clips" into the swivel stud's hole, just as it would for all other non-AR rifles. The bipod pieces that clip onto the adaptor holes have a set of holes in them to accept a standard 1" or 1.25" sling swivel.
Good luck...squibload