I've finally started using multiple sheets in Google spreadsheets. I found I was creating multiple spreadsheets, for tracking different items, but all containing the same tracking columns. So, I put them into one spreadsheet, and created one master sheet to track running totals compiled from each sheet.
So, if you need to do this, and want to reference a value in another sheet, it's actually quite simple. You just need the name of the sheet, and the cell location. So, if the sheet was named "Sheet2" (creative, huh?!), and the cell was C3, you could reference the value from any sheet with this formula:
However, I ran into one error, when I had an ampersand ("&") character in my sheet name. It gave a 'circular reference detected' error. I guess it thought the "&" was part of the equation. So just use alphanumeric characters when naming your sheets, and you should be fine.