It's pretty simple:
<T4, replaced by L68 BoE greens

T4, mostly replaced within the 1st zone.
http://wotlk.wowhead.com/?item=38237 for example is as good as or better than anything you get from T4.
T5, mostly replaced within 3-5 levels.
http://wotlk.wowhead.com/?item=35576 or
http://wotlk.wowhead.com/?item=41181
T6, will be mostly replaced by late 70's, or L80 quest drops. Beta only goes to L77 right now so we don't have a lot of item data yet.
Sunwell drops will certainly be outdone in most cases by the 1st tier raid drops and in some cases by L80 noob blues or epics. Look at the BoE L80 crafted 2H, blows away anything in the game now :
http://static.mmo-champion.com/mmoc/...ldestroyer.jpg
Some TBC items will stick around for a while, the niche or particularly well-itemized drops, or things that just don't have a replacement. I remember wearing my T2 belt until L67 or something just because I either missed a quest, chose the wrong quest reward, or didn't get a boss drop. And some things are itemized in an OP'd way so they'll be good practically until the *next* expansion -- example, Styleen's is one of the best tanking trinkets *still* to this day.
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