Apple engineers during the technical sessions of WWDC 2010 conference in San Francisco recommended the software created by Klokan Technologies.
What did they say? You can watch the video (available after free registration) at http://developer.apple.com/videos/wwdc/2010/.
James Howard, Software Engineer Map Kit Team, Apple Inc. on the stage of WWDC 2010 in the USA said: "Gdal2Tiles is a really great utility. I recommend using it."
Apple even officially published a source code for an iPhone application, where you just drop in the generated map tiles, to get an instant offline viewer: