D90- plastic sensor

People,

I am planning to buy a Nikon D90 this week, but my friend who uses (Canon 40D) is emphasizing not to buy the same. One of the drawbacks he claims is the D90's sensor, which is made up of plastic. I would like to know will this be a major drawback, for me not to go for D90.

My friend also wants me to either go for Nikon D300s or Cannon 7D. But as this will be my first camera, i would rather like to invest more on lenses than on body.. and later (may be 18-24 mnths) go for any new body...

Kindly suggest..!

AJ