Yesterday, I downloaded Unity free Indie edition. I also experienced the crash that your_mum had. I am using Windows Vista 64, Quadcore CPU @ 2.66GHz, 6 GB RAM, ATI RADEON HD 3650.
However, I found a workaround. Just remember to save often. Before I add any trees to my terrain, I click Terrain->Mass Place Trees and I add a small number of trees (1 to 5, something like that). It then adds the trees. Then I can click on terrain in the hierarchy, then click on the place trees tool in the inspector and tree placement works! You can then add more trees of differing types, placements, etc. Just be sure to save often, because I did experience one other crash.
However, by using the Mass Place Trees trick the crashing is minimized. I think this is most certainly a bug since it crashes to desktop and I can reproduce this regularly. I will look into submitting a bug report.
If any of the developers happen to read this, please feel free to ask any additional questions.