Hi Dread,
Nikon "G" type lenses do not have an aperture ring because the aperture is set by most digital camera bodies now (I think Canon has been like this for a long time?). With the normal "D" type lenses (like the 60mm Micro Nikkor) that have an aperture ring you have to set it to it's smallest aperture when mounted on the camera and then use the camera dials to set the aperture, not the lens. The aperture ring was continued on the "D" type lenses for a long time so they were compatible with old Nikon film cameras where the aperture was set on the lens, not by the camera.
With lenses in conventional use this is not important, the one time the aperture ring is useful on a Nikon though is on the shorter Macro lenses like your 60mm, where if you put it on extension (like bellows or tubes) to work far above 1:1 it is best to reverse the lens. The advantage in this case of Nikon over Canon lenses is the Nikon lenses stop down to their smallest aperture when taken off the camera, so you can then use the aperture ring to set what aperture you require. With Canon's I believe they fly open to maximum aperture off camera and you need an expensive bit of kit to fix on the bayonet to control the aperture when reversed.
One other unique feature about Nikon's is your camera shows you what is the effective aperture your lens is working at, not the relative aperture marked on the lens. I have had my D200 indicate my 60mm Micro Nikkor was set to an effective aperture smaller than f32, which is the lenses smallest aperture.
This of course was true because the lens was far enough extended so its effective aperture was less than the minimum marked on the lens. No need to do the sums for added extension therefore to find if diffraction could be a problem because the camera itself, unlike other makes that simply show only the relative aperture marked on the lens, had already worked it out for me.
The only Nikon a lack of aperture ring on the Sigma (or Nikon "G" lens)would make it incompatible with would be some of the old Nikon film cameras. In fact the D40 and D40X are the first Nikon's that can only work with the latest "S" type USM motored lenses and cannot use the older "D" lenses like your 60mm Micro Nikkor as they do not have the motor in the body needed for the older "screwdriver drive" type lenses.
For a key to the Nikon codes see:-
http://www.kenrockwell.com/nikon/nikortek.htm
I don't know the Sigma 150 but it gets good reviews:-
http://www.photozone.de/8Reviews/lenses ... /index.htm
When you get old like me Arlon you have the sense to stop inside in the warm and dry in front of a computer instead of going out taking photographs in bad weather!
DaveW
I use a Canon 30D with a 60mm macro and get pretty good images from it, though the 60mm does not have that long stand off of my 100mm macro but still provides me with that 1:1 ratio. I am presently considering a Sigma 150 macro for an even greater stand off, so as not to spook things away, such as butterflies and other skittish insects and maybe some small animals. 
