Commercial Bluray Movies Single or Dual Layer?

I haven't taken the plunge to upgrade all of my components of my home theater system to bluray and as of now I still do not own a bluray movie, but I am hoping that someone who has examined multiple bluray discs would be able to tell me if most of their discs are dual layer bluray discs (50GB) or single layer bluray discs (25GB).

I noticed that in the early days of DVDs, most commercial DVDs were only single layer. I am curious to find out if Blurays are following the same trend.

This matters because I plan on making backups of any bluray movies I buy (to protect them from my children) and single layer discs are significantly cheaper than dual layer discs. When I make the plunge to bluray, should I buy more single or dual layer discs.

Thanks for the help in advance.

Thanks for the replies, I appreciate the info. Looks like I will have to wait a little longer until dual layer bluray media drops in price before I start planning on backing up every bluray movie I buy.

My kids aren't too bad with DVDs, I am just ultra-paranoid, and when blurays are $20-30 a pop, I get even more paranoid. It will certainly be nice four years from now when bluray movies are on the $5 shelf at Walmart (if Bluray follows the same product cycle as DVDs.)