Remember The Milk offers several methods to integrate your task list in Google Calendar. One of the methods is to put a task gadget in the sidebar (it's the same gadget you can embed in GMail as well). Another method is to add a special calendar, which results in checkmarks at each day. When you click those, a popup appears with all tasks due for that day. The final method which is relatively easy to find is in the Info tab of the Settings page. There you'll find a link pointing to an iCalendar resource, containing all your tasks (possibly represented as events).

These methods are not fine-grained enough, I'd like to control which tasks should be visible in my calendar. For instance, I'd like to see major deadlines (which could be quite far in the future). What I don't like to see are minor domestic tasks such as watering the flowers.
It is not difficult to achieve this. It boils down to create a special list and add the URL of the iCalendar representation to Google Calendar. These are the steps in more detail:

1. Tag all tasks which you would like to see in Remember The Milk, for example by giving those the gcal tag.
2. Create a special list: enter the search query tag:gcal and click Save at the right side of the screen. You're free to use a more sophisticated query, for example by excluding "sleeping tasks" (explained here in more detail).
3. Go to that list, make sure no task is selected.
4. In the box at the right side of the screen, you see a link iCalendar (Events). It is important to have the Events version in order to see your tasks properly. Copy the URL to the clipboard (usually right click on it and choose something like Copy URL).
5. Go to Google Calendar, at the left you see your list of calendars.
6. Click Add and then Add by URL.
7. Paste the link from Remember The Milk and press Add Calendar.

You're done now, but most likely you won't see your tasks. This is because Google updates your remote calendars only once in a few hours (see here). The lack of realtimeness is a bit of a downside, but it's better than having my calendar overloaded with unimportant tasks.