I've had the same problem with iCal alarms not showing up in Snow Leopard. Yesterday I finally found the source of my problem, and the solution.
It turns out that alarms were not showing for alarms in my Google calendar. This explains why some alarms wouldn't show (Google calendar items) while others would. It has to do with the way iCal sets up Google calendar accounts in Snow Leopard, which differs from the way it was done in Leopard. This also explains why alarms work fine on one of my iMacs, but not on the other. On the iMac that does show alarms, the Google account was set up on a previous version of the Mac OS's iCal (I don't remember which one) and was then ultimately upgraded to Snow Leopard. On the other Mac, the Google calendar account was setup in Snow Leopard.
The solution to my problem was simple. I deleted the Google account from iCal and set it up again, this time using the old way to do this, not using iCal's procedure for setting up Google Accounts.
To do this, open the Preferences in iCal, go to the Accounts tab and click the + to add an account. Select CalDAV as account type and enter the user name and password of your Google calendar account. The server address you need to enter, depends on whether you're going to sync your primary Google calendar or a secondary one.
To sync your primary calendar, enter the following url:
https://www.google.com/calendar/dav/youremail@gmail.com/user
...replacing
youremail@gmail.com
with your actual email address.
If you want to add a secondary calendar, instead of inserting your email address in the Account URL as described in step 4, you need to grab the calendar ID for that specific calendar. You can find it by opening Google Calendar, clicking the arrow next to the calendar you want to sync, and selecting Calendar settings. In the settings, find the Calendar Address section near the bottom of the window, then just copy and paste the Calendar ID (which is formatted like an email address) in place of
youremail@gmail.com
in the URL above.