The Tamron is a fine choice. but personally i find that for macro one needs to really focus manually anyway, so why not look at manual lenses which can be obtained much much cheaper and have fabalous results.
The flispside is that you lose metering.
This is a huge problem initially but as you get used to it, I think it is worth the extra effort.
The Nikon 55 micro 2.8 and 3.5 ais and ai are cheap but fine choices.
If you need more working distance, the nikon 105 micro Ais and the tamron 90 mm manual are excellent choices too.
Also though not a macro lens you will be surprised what nice close-ups (not true macro) a $80 nikon Series E 75-150 can provide.
In fact you can mount even pre-Ai lenses on a D5000 which can be bought REALLy cheap.
The money you save, can be put towards a tripod (you will NEED one for macro), flash or some other lenses.
just my two cents, but hey if you have the money, go for the autofous option.
Though I am not very good, i seem to have more fun with the manual lenses.
