While helping a student the other day, I stumbled across a background ESRI now offers in ArcGIS 10 with continuous world imagery. Similar to Google Earth, it streams the orthophoto tile to your system and selects the most appropriate photo and resolution based on your view extent. While it does require internet connectivity to work, using this layer can save the time of downloading potentially hundreds of aerials for large study areas.
It is not as sophisticated as Google Earth (you get one image of ESRI’s choosing, no scrolling through history), but for simple tasks it’s great.
A word of advice when using. Turn the layer off until everything else on your map is set up, then enable the background layer. It’ll make your processes run much faster, since the map won’t need to download and re-render every time you change your view.
Some tips on projection. These background files are in the web mercator projection (see link below), which is not compatible with the UK datum. Remember to set a transformation in your map document properties to correct this, or you’ll be ~half a kilometer off.