There's been precious little actual HD on our latest arrival, but my daughter has just stumbled upon the second of the Christopher Reeve Superman movies on it this afternoon. Great picture, but boy do those "special" effects look ropey in high def! Stil, Sarah Douglas, a teenage crush of mine, looked mighty fine. A shame she's only really done animation voice work and the odd TV drama since.
Finding Nemo is also showing this afternoon on Five. However, be warned that in the normally reliable Radio Times it doesn't say it's showing in HD - it is (and looks fab).
If Five on channel 150 doesn't have an "HD" top right, it's only showing upscaled versions of movies. Alternatively, check the information box in the EPG for the [HD] tag.