Okay an interesting problem; I assume related to some of aaime's comments about raster resampling.
I just updated uDig to use 8.0-M0 and my usual test of a shapefile+geotiff failed (see attached image).
1. The bluemarble.tiff raster is displayed; but only 1/2 of it is displayed.
2. If I zoom out or in is fixed
3. importantly if I set my map coordinate reference system to be the same as that from the geotiff it is fixed. In this situation they both are exactly the same and the display is correct.
4. if I set it back to the co-ordinate reference system from the shapefile is is rebroken.
The uDig sample data "bluemarble.tiff" and bc_borders.shp are shown.