Hi, Pete,
To the best of my awareness, the capabilities of the FindAll method in the List class (to use a predicate/delegate or an anonymous method) don't extend to the dataset. So like John Baird ssid, you're basically confined to using RowFilter or Select(). Obviously, you could pump your dataset into a List class and do it that way, though if it's a large dataset, there will be some overhead.
That's one of the goals of LINQ....to have a uniform method of querying, regardless of what the source data is.
Kevin