The 256 default DataSeries lookback cache is NT7-only, it's one of the things they did to reduce the memory footprint and improve performance over NT 6.5.
It's a great improvement, only problem is that up until now if you referenced back >=256 bars it would return bad data without any warning. They now say that the next beta will throw an exception, so it should all be good then.
You can still have inifinite lookback if you want by asking for it when you create the DataSeries:
DataSeries myDataSeries = new DataSeries(this, MaximumBarsLookBack.Infinite);
There's also a property for "Maximum bars look back" in the property sheet when you add an indicator to a chart, or configure a backtest/optimization/walkthrough, which defaults to 256; you can set it to infinite at that time.