2 machines here:
My first machine is a m6805 purchased on 1/25/04. I don't know when the hinge covers cracked because I never noticed them until I saw a post here about the cracked hinge covers in May 2005. When I checked my hinges, sure enough, the left hinge cover had cracked about 60% of its width. Didn't worry about it too much; it was just the hinge cover, and I needed the machine for work. Besides, the machine was perfect in all other aspects.
My second machine is a m6811 purchased in Sept. 2004 (made in July 2004). The hinge covers cracked in less than 2 weeks.
So, in Nov. 2004 I sent the 05 back to get the hinge covers fixed. Two weeks later, I had my 05 back in pristine condition. No further hinge cover problems in the last 6 months.
In Jan. 2005, my m6811's motherboard went flaky and started causing frequent boot failures and shutdowns. Sent it in for repair the last week of Jan. 2005. This time it took 3 weeks before I got a replacement machine back (different serial number, refurbished, also made in July 2004). I guess it was beyond repair. Anyway,no machine or hinge cover problems since mid-Feb. 2005.
I do not notice any difference in how tight the hinges are, so eMaches/Gateway/Arima must be getting stronger hinge covers now.
Hope this helps!