I don't want this to sound like damage control, so first of all I will say that if MS has done an awful job keeping information regarding the industrial design of the console a secret in the last month.
However, there are several factors stacked against MS from having a clean "surprise" showing at E3 2005 compared to previous console unveilings. Several of them they can only blame on themselves:
1) Huge increase in interest by the general public in gaming (so new systems are awaited with much more anticipation, increasing the chances of pre-launch leaks)
2) Continued proliferation of the internet, camera phones, and general information sharing technology
3) Microsoft's desire to have a tremendous announcement this May unparalled to any console announcement in history. When the PS2 was announced, hardcore gamers took notice and waited with bated breath, but few casual gamers knew anything of the PS2 until it was already a success in the marketplace. Now with the X360 announcement, MS wants all consumers interested in entertainment (gaming or otherwise) to take notice. All major gaming and non-gaming publications already have access to screenshots, final hardware renders, logos etc. much sooner and in greater quantity than they have in the past. Oh, and let's not forget the 30 min informercial they've secured on global television. That introduces hundreds if not thousands of potential leaks.