I'm guessing now, since I don't work at Google I really don't know anything :)
Since this is a fairly common question in the Google Support Forums for WM, here are my 5 cents.
Windows Mobile doesn't support multiple Calendars, it is really as simple as that.
If you would sync 2 or more Google Calendars to your device they would end up in the same calendar database.
"What about Categories" I hear you asking.
Even if they did find a way to translate the multiple calendars to multiple categories, and I'm not sure how they would do that without combining the calendars already on the server, there is always the problem with translating them back again. Otherwise you would end up with just one Google Calendar, and this you can do today but it kinda kills the whole idea with multiple calendars doesn't it? Only way I can think of is to have a one-way sync, two Google Calendars -> One WM Device. In my opinion that's not a good solution.
To prove my point:
Consider what happens if you have an ActiveSync relationship with a server (Exchange or Google doesn't matter) and you sync the device with outlook, that's right: You will get exactly the same calendar in both places no matter the origin of the event. The same would happen with multiple Google Calendars, they would be duplicates of each other.
The difference in Android for example, is that you actually have the possibility to have multiple calendars on your phone, displayed combined yes but still separated (or just the ones you are interested in for the time being). Don't know about the Iphone but I guess it's something similar.
So in my opinion you should all hope for Windows Mobile XX (since they left it out in 6.5 as well). Since this version was rumoured (I haven't seen any details yet but according to early adopters they left it out) to have support for multiple ActiveSync relationships this indicates that you would have support for multiple calendars. Why Microsoft didn't make this happen is...a mystery. Windows Mobile is left behind, and not even the latest version stand up to Android or iPhone. In the meantime you have too look for 3rd-party solutions that uses one of the following methods:
Separate application on your phone. OggSync uses this and it works out nice. The translation between Categories and different calendars are then made by this application. This is not a free service. http://oggsync.com/
Server side translation. Used by Nuevasync and is a quite neat solution, you use ActiveSync and really doesn't notice any difference to a normal sync. Free service! www.nuevasync.com
Since GooSync fail to let me know anything without signing up, well I wouldn't recomend signing up for a service that they won't explain.
Confusing, understandable? Ah I did my best :)