No having a Lion preview I can't say for sure, but I would imagine this will look odd if windowed along side other windows with a different UI. However the push from Apple appears to be for full screen apps. You'd simply switch your app and the whole screen shows what your working on.
In full screen mode I can see how this iCal UI would work, it will be like having your whole screen as a calendar. I can only really see an issue when working with side by side windows. No matter how much Apple push thier ideas, some people will prefer to work this way.
To be honest I couldn't care what it looks like. Its the abilities of the programme that matter to me. If there are new features introduced that make iCal a better programme then great. The UI is ultimately just superficial to the abilities the programme offers the user.