The most important change in UV editing since 2.45 is the removal of Face/UV mode, previously you had to exit editmode before you could see what your UV textures looked like, now you can see your UV textures in editmode as well as change the UV coordinates.
This is a draw mode option for solid drawing so you can see textures on a solid shaded model.
This is especially useful when you want to edit a mesh while seeing its textures.
(SolidTex in the 3D View panel or Shift+T)
When editing UV's for tiling textures its useful to see the image under the UV coordinates. This also helps to see where UVs run over seams in the pattern.
Textures for games or realtime display may need to be rescaled to 512x512 or 1024x1024 so the graphics card can load it efficiently, changing the original aspect ratio.
Displaying non square pixels helps when editing UVs over these textures, especially when you need to rotate.
The default drawtype has changed from dashed lines to grey lines with a black outline and highlighted edge selection.
Other drawtypes can be useful when mapping over different images.
UV Stretch display can help you find areas in your texture mapping that have the most distortion.
The area and angle differences between the 3D faces and the UV coords are used to color faces in the UV view.
Red for maximum stretch, blue for none.
The UV editor now has a 2D Cursor, like blenders 3D cursor it can be placed on left clicking and numerically in the "View Properties' panel.
This also functions as a pivot for rotation or scaling and can be used to Snap to the selection.
Enabling "UV Local View" from the view menu will only show the UVs for the active faces image.
Models with multiple image textures make it hard to tell what UV islands belong to what image. This is especially useful for when "Sync Selection" is enabled.
The option is accessed from the mesh icon in the UV/Image header.
UV Calculations such as unwrap now take into account the image aspect ratio when projecting and laying out unwrapped uv islands.
This is useful when projecting UV's to map a non square photo to and when the texture for LSCM Unwrapping is not square.
This can be disabled in the editbuttons "UV Calculation" panel to work as before.
The snap menu was added to take advantage of the 2D cursor. "Selection -> Adjacent Unselected" helps when adding filling in faces between UV mapped faces.
This tool averages the size of separate UV islands based on there area in 3D space. This can be useful when joining objects that will need to share the same texture. (Access from the UVs menu and Alt+Akey)
