It's not difficult to walk around China and spot poor translations, which makes it all too easy a target. For this reason, I'll try to keep this kind of thing to a minimum. However, this sign, in Qilihe District, was too good too pass up. Thanks to Jess for the spot.
梦想 (mèngxiăng) simply means 'to dream', but dictionaries list a second meaning 'vainly hope', which suddenly renders the dream delusional, futile, and ultimately unattainable.
While it's true that dreams can be associated with events you think are unlikely to happen ("Win the lottery? Are you dreaming?"), other kinds of dreams are quite plausible ("I'm dreaming of a white Christmas"). My feeling is that this last one is the kind of dream in 梦想中国.
And, of course, the Olympics slogan is "One world, one dream" (同一个世界，同一个梦想).
I think the reason I like this sign is because it's an expression of modest wishes, which is refreshing in the climate of patriotic rhetoric and unabashed sloganeering which surrounds these Olympics.
But, for what it's worth, I hope the Olympics are a great success. China has put an amazing amount of energy into it. Not long to go now.
So, a better translation? How about: "China's dream is for a joyous Olympics"