You might want to check out Ascent from Montbello Software. Mac native and runs circles around Garmin Training Center for the 205/305. I'm using it for my 405, but only after exporting a file in windows from GTC and importing it into Ascent.
That is the mistake I made.... older model 205/305 will work on Garminconnect in May. The 405 is not slated for support until Q3 of 2008. You only find this out after purchasing and downloading the software. For now I am using Fusion (same as Parallels). Also, keep an eye on runnerplus. They are very close to supporting Garmin and have a very robust site.
The Garmin 305 already works with Macs. I've had mine for about 6 months and have been using it exclusively with OS X. I use Garmin Training Center. The 305 will not be compatible with Garmin Connect until 5/21 but that's the device not the OS.
It is only the software that neds updating. I know the software for macs are supposed to come out in the fall, but the 405 has been beset by delays. So you can buy one and EVENTUALLY it will be supported by Mac, I just can't tell when by anything I have seen Garmin put out.