From this Post:
http://www.avsforum.com/avs-vb/showt...946618&page=48
Mine has a lot of dust blobs (>7) at about 35 hours and my room is not very dusty. Only two are visable during normal viewing - one red one and one blue one. Mitsubishi and Projector People are replacing it as defective (new one should be here on Thursday).
You may not want to do this as it will show dust any unnoticed dust blobs... but if you want to check if you have any dust blob here is how you can do it:
1) Display a black screen by going to Installation -> Back Color -> Black. Then switch the projector to a non active input.
2) Adjust the focus to its maximum "left arrow" position. If you have dust blobs and or particles they will show up as the color of the panel the dust is on.
3) Adjust the focus to its maximum "right arrow" position. If you have any dust blobs and or particles they will show up as the color of the panel the dust is on.
On my projector lots of dust particles show up on at the maximum "left arrow" position and only a few show up on the maximum "right arrow" position. As I stated before only two show up in the focused position.