I think we did consider .Ago(), but we felt it wasn't as clear as LookBack and that the difference in length wasn't very important with autocomplete.
RE is not actually pushing for 1.0, we released 1.0 in the summer of 2007. We've had one edition so far with array indexing disabled, so we might be able to go back in a future edition. However, that would break existing systems again, unless we left both methods in. Since the ISeries interface is also for customers to implement if they need, they would have to write two methods that did the same thing.
When we switch to .NET 3.5, you would be able to add en extension method to use Ago() instead of LookBack() if you wanted. It would also provide us a cleaner migration path back to indexers if we chose to go that route, since we could switch the interface to use indexers and provide a LookBack extension method which would enable most existing code to keep working.