I can't speak directly to the Mac OS X versions, as I am a Windows user. I have used both the Spyder3Pro and the Spyder3Elite, with the older 3.x software and the newer 4.x software. With all combinations, one thing I have noticed is that the Spyder software actually loads your profile when it starts. When possible, it actually loads and syncs the profile with whatever hardware devices support ICM directly, which seems to be the case with nVidia cards and my Apple CinemaDisplay 30" screen.
I have manually set my color profile with the Windows ICM control panel, and the Spyder software always overrides it. In the case of my desktop workstation, the Spyder software loads and installs the profile instantly, so I rarely ever see the switch. With my laptop, however, the Spyder software seems to lag, and there can be up to a couple minutes of time between when Windows starts and the calibrated profile is actually applied.
If the Mac OS X version of the software works in a similar way, I would assume that manually setting the profiles would not work unless you also disable the software. The Spyder3 is an active calibration device, and if you keep it on your desktop, it will actively measure the ambient lighting and alert you when the profile is no longer accurate. The newer version of the device and the 4.x software also seem to actively adjust brightness to maintain your target screen luminance (I try to keep mine around 80-90 cd/m.)
If you really want to calibrate multiple devices, and have multi-device profiles, I highly recommend getting at least a Spyder3Pro (which isn't terribly expensive, around $140) with the v4.1 or later software. The new software brings a ton of new features to the table, making the Spyder3 competitive with much more expensive higher-end calibration systems like X-Rite's EyeOne (i1) system. The 4.x software does support multi-device profiles as well as multiple screen profile normalization, allowing you to create profiles for multiple displays (such as a workstation, laptop, projector, and TV) and ensure that your images look the same across all devices.