You can do what you're asking several ways:
Using Google's spreadsheet C# library (as in Tacoman667's answer) to fetch a ListFeed which can return a list of rows (ListEntry in Google parlance) each of which has a list of name-value pairs. The Google spreadsheet API (http://code.google.com/apis/spreadsheets/code.html) documentation has more than enough information to get you started.
Using the Google visualization API which lets you submit more sophisticated (almost like SQL) queries to fetch only the rows/columns you require.
The spreadsheet contents are returned as Atom feeds so you can use XPath or SAX parsing to extract the contents of a list feed. There is an example of doing it this way (in Java and Javascript only though I'm afraid) at http://gqlx.twyst.co.za.