Finally my original answer is completely obsolete.
Google updated features with Alternating Colors features in "Format" menu item. There you can easily select a default scheme and customize as you like:
Be sure to remove your old manual formatting, as this doesn't automatically replace/override.
=== OBSOLETE BELOW THIS LINE ===
Google added this feature now in the "new spreadsheet format".
This can be enabled for new documents like follows:
On "MyDrive" go to settings choose Editing and then "Try the new Google Sheets".
Please be aware that as of now not all features are complete, so treat it as a beta version.
One of the changes in this new spreadsheet is the possibility to define custom formulas in the conditional formatting option.
This formula would then allow the alternating coloring in the sheet (if applied to whole sheet):
=MOD(ROW(A2),2) < 1
Update. The following will also work (thx @Qwertman):
=MOD(ROW(),2) < 1
Just for completeness: I'm using the new sheets now for quite some time and did not yet find a feature I was missing. While the custom formulas are very helpful - not only for alternating colors.
Select Conditional Formatting:
Create a custom Formula with Background Color and Range of Cells:
See the result: