A recent update to the official Google Notifier app has added a slick (and apparently Mac-only) feature that should make it easier to add events to Google Calendar while you work. A new "Quick Add to Google Calendar" contextal menu item does exactly what you might think - it allows you to right-click a selected piece of text in most Cocoa apps and create a new event in gCal from it, set for the specific time you used said option. Now this might not offer the flexibility of the Google Calendar Quicksilver module we mentioned, but it's a really handy way to rapidly fire off events based on information and text you're already dealing with throughout the day, and you can easily visit your Google Calendar to organize and manage their specific details at your convenience.
There only way I caught this update, however, was downloading what I perceived to be the latest Notifier from Google on a new machine. Strangely, upon first running the Notifier, I was prompted to download an update which is what brought on this new feature. If you haven't been prompted yet, try restarting your Notifier to see if it kicks the update process into gear.