There are a number of requests in regards to rolling up calendar events from various sites to a site hosting a master calendar.  The most economical choice is to use SharePoint Designer 2007.

 "This post, only, focuses on creating linked calendars." 
1.  You will need to open up the site that will host the master   calendar.  After selecting the URL for the hosting site, Click on the Open button.

 
2.  After the web site is opened in SharePoint Designer you will see all related folders and files displayed in the Folder List.

3.  Click on the Task Pane menu and Select the  Data Source Library. The Data Source Library is displayed.

4. From the Data Source Library task pane, select Connect to another library... the Manage Library dialog box appears.  Click on the Add button.

5. The Collection Properties dialog box is displayed.  In the Display Name text box, type the name of the site.  In the Location text box, type in the URL for the source site.  Click on the OK button to close the Collection Properties and Click the OK button to close the Manage Library dialog box.

 Note: The new Collection - IT Services - is available from in the Data Source Library.

6. In the Data Source Library task pane, navigate to the Current Site Collection .  Click the plus symbol next to Current Site.  Next, Click the plus symbol next to the Linked sources and Click Create a new linked source… button. Next, Click on the Configure Linked Source... Button.

7.  The Link Data Sources Wizard dialog is displayed.  In the Available Data Sources list box, ensure that the Current Site Collection data sources are displayed.  Next, Click on the SharePoint Lists and select the Calendar list.  Next, Click on the Add button.

8.  In the Available Data Sources list box, Click on the IT Services Collection's data sources are displayed in the Available Data Sources.  Next, Click on the SharePoint Lists and select the Calendar list.  Next, Click the Add button.  Finally, Click on the Next button.

9.  In the 'Select the link type that best represents the relationship between the selected sources' section, Select the option, 'Merge the contents of the data sources.  Choose this option if you'd like to sort, group, and filter the sources as one long list.'  Click the Finish button. 

10.  The Data Source Properties are displaying the added Calendars in the Component source properties section.  Click on the OK button.

Note: After the Linked Data Source is saved, it is named New Data Source in the Linked sources section.

11. Using SharePoint Designer, Create a new ASPX page in the current site.  Click in the Form control to position your cursor.

12.  Click on the Data View menu.  Next, Click on Insert Data View.. option.  The DataFormWebPart is added in the Form control.

13.  Navigate to the Data Source Library task pane. Drag and drop the merged calendars - New Data Source - onto the DataFormWebPart on the new page.

14.  Click on the 'On Object User Interface' (OOUI), It will open the 'Common Data View Tasks'.  Click on the Edit Columns.. option.

15.  Remove both Modified By and Modified from  Displayed Columns list box and add Location field from the Available fields list box.  Next, Click on the OK button.

16.  Navigate back to the Common Data View Tasks properties.  Click on the Filter option.  Use the table below to configure the filters. After configuration, Click the OK button.

17.  Save the page and preview in the browser.  You can add events to all calendars included in linked sources and it is reflected in the rendered page.
Note: You can attach the page to the master page for the site to ensure that it is branded like all other pages.