Came across a new upsizing technique yesterday:
Tried it myself last night upsizing an image to 200% and comparing it to results achieved with
a) stair interpolation (as implemented in Fred Miranda's actions)
b) upsizing at the point of RAW conversion (using Photoshop ACR)
c) Genuine Fractals (ver. 3.5)
All images worked on was a 16 bit tif converted from a D60 RAW
The new technique is the best yet in my opinion - no its not perfect, but will provide a very acceptable 20 inch print at 300DPI from a 6mp sensor - or 40 inches at a more realistic "large print" 150 dpi.
Unfortunately the new method is only available to Photoshop CS users.
For non CS users, in 2nd place comes upsizing at RAW conversion (if your conversion software allows this), followed by stair interpolation, with Genuine Fractals following slightly behind (though some may prefer the GF look, which is cleaner than stair interpolation, I think it adds too many artifacts).
In fairness to GF, though, at sizes ABOVE 200% I think this starts to come into its own. At 400%, none of the results were pretty, but GF was probably the best of the lot.
I have not tested this with jpg images (and don't intend to) since for quality interpolation the starting point must be as clean and artifact free as possible.