@Ruleslawyer:
There is definitely some truth to what you say but not much is actually relevant for miniature photography in contrast to macro photography. We're not generally bother by extreme corner sharpness or being able to micro adjust the focus, so a macro rail is overkill when taking a typical photograph of a mini. Even more so considering we generally try to use smaller apertures and smaller apertures means larger depth of field and increased both general and corner sharpness until the effects of diffraction starts to kick in. I think "close up" photography is closer to the truth when it comes to miniatures then macro or micro.
The Sigma 28mm 1.8 you mention is probably a great buy for miniature photography given its properties, but is actually more expensive then a Sigma 50mm macro lens or a cheaper zoom that would also do the trick. You also do not need a fixed focal lens to have a cheap lens that can focus close and it is not typically "1:2 instead of 1:1" manufacturers put the macro or micro stamp on many lenses between 1:1 - 1:4.5 (approx.). The Sigma you own for example has a reproduction ratio of 1:2.9. Many zoom lenses are as capable and significantly cheaper.
I ran a test little more then week ago with a few lenses. Canon 85mm 1.8, Canon 50mm 1.4, Sigma 50mm 2.8 (macro), Sigma 17-70mm, Samyang 85mm 1.4, canon 28mm 1.8. I was able to get decent results from all these lenses, save perhaps the Samyang, on a space marine subject. The 85mm, all manual, Samyang cannot focus very close at all and this kind of work is kind of the antithesis to what it's supposed to do. I could hardly focus on the mini in the viewfinder (my Canon 30d doesn't have liveview)... but still I wouldn't be very ashamed of having this image in my gallery:
So this is the worst, in focus, shot I got from the series. Sure, not much better then most compacts but on the other hand many compacts can take great close-up photography. A compact is a better buy then a dslr (interchangeable lenses) when it comes to miniature photography in that it gives you more bang for the buck. I'd actually recommend anyone not seriously into photography to buy a good compact camera instead.
The beautiful subject is a Space Marine Captain by Ben Jefferson from my collection.
I suggest the macro rail simply because the non macro lenses are pretty touchy in manual focus. It may be overkill, but its a relatively cheap solution compared to going for a full macro lens.
As for zoom lenses- they are never as capable. It might not matter than much for minis, but a prime lens will almost always be sharper unless you're comparing pretty extreme ends of the spectrum in favor of zooms. For web size images it might not matter much, but the difference between a cheap prime and a cheap zoom is fairly significant.
The main reason I like the 28mm is that given an APS-C format camera like most consumer DSLR's you're getting a lens that is very close to 50mm equivalent. A 50mm prime is cheaper, but its a slight telephoto lens (~75mm equivilant). Most people choose their first prime to a neutral, fast lens. This makes it a really good dual purpose lens. Plus it is a EX series lens. The build quality is better than the cheap 50's. Not that there is anything wrong with a cheap 50mm, but you're not paying more just to pay more. Either way, if you go with a 50mm or a 28mm prime, I doubt you'll regret having a really fast lens in your bag.
If I were to recommend a lens strictly for minis I would just suggest the sigma EX 105 f2.8 I own. Its an amazing lens, but very limited in scope, and probably way more than most people want to pay.
I didn't really consider the compact cameras. The OP already said he wants a DSLR and is looking for a lens. There are some that will do the trick, but good macro photography is all about manual controls, especially the focus. Most compact don't have many manual controls, and those that do almost never have focus controls. By the time you get a really good point and shoot, you're looking at the price of low end DSLR kit, or maybe one a year old. Compacts are great if you only can have one camera and don't want to lug a DSLR around. However they're a 'good enough' solution and don't really have a place in a discussion on lens selection.
All of this is a little about pixel peeping though. Any lens with a macro function on a DSLR will take fine quality shots for web use. Even many compacts will take okay shots if you can get them to focus where you want. The whole light setup and using a tripod properly will contribute a lot more to having professional looking pictures. I know I've been pretty lazy about getting a proper light setup myself. I have a light cube in the mail that should up the quality of my photos more than any lens would.