Here is the most helpful site I found:
http://www.tuaw.com/2009/01/07/hardware-that-supports-iphoto-09s-geotagging/3#comments
I've been playing around with geo-tagging my aperture collection for the last few days and currently the two best programs I have found to do it are HoudahGeo and GPS2Aperture. If you are using iPhoto '08 then HoudahGeo seems pretty easy but then again, you are a candidate for iPhoto '09 so it doesn't matte. For aperture folks these programs have major shortcomings. My own unofficial testing revealed that HoudahGeo doesn't work except for referenced masters (which I don't exclusively have) and is a little buggy. GPS2Aperture is also buggy but does work with aperture masters in the library, however, it is much more cumbersome to use and assign geopoints to making it unfeasible in my mind.
Notably, if you are going to get a separate GPS unit and then import the trackfiles to log with your pictures then you should know the above limitations of these programs (which accomplish this task) and their integration with aperture. However, GPS2Aperture would likely be easier to use with an actual GPS device.
Personally I think the best solution at this point is remake my library exporting my photos into folders and then re-importing them as referenced masters for use with HoudahGeo but as this seems very cumbersome I am going to wait until the next aperture update to see if this feature is added. If not then I will have to re-do my whole library for this feature. How can iphoto have more features than aperture? Ugh.
EDIT: I just used maperture and it works pretty well with aperture. Only annoying thing is that I can't figure out how to see all my photos on one map but that's find for now.