JOSM and Merkaartor are really nice programs for editing OpenStreetMap data on the desktop, and OSMTracker is acceptable for Windows Mobile devices. However, OSMTracker is very inflexible when it comes to tagging abilities. So a perfect mobile application would combine the best of both worlds:

• displaying the raw XML data, maybe in real-time via mobile internet connection
• displaying the rendered map
• recording a GPX track
• take pictures if the mobile device features a camera
• serve as a voice recorder
• adding and tagging nodes, ways, and maybe relations in a key=value manner
  • maybe allow presets for different objects, like JOSM has. These could be implemented like the customizable buttons in OSMTracker.
  • auto-completion for keys and values would be great for people like me who cannot remember actual tag names...
  • The absolute killer feature would be the ability to automatically import the changed data into desktop software like JOSM/Merkaartor, and export diffs.