Tech trading is being replaced by cooperative teching. This will work as an effective replacement for tech trading if diplomacy works well in Civ 5. However, in my opinion diplomacy hasn't worked well in any of the previous Civs and I don't know if I trust Civ 5 to succeed where the past 4 Civs have failed.
Granted, tech trading is ultimately a diplomatic mechanism. However, it's a mechanism with a very obvious payoff model making it easy for the AI to figure out if the trade is worthwhile. It also doesn't require the two civilizations to be on good terms. Cooperative teching, however, has a slightly more ambiguous payoff and will only work if civilizations are on good terms. I fear that, due to a lousy and random diplomacy in Civ 5 (like past Civs), my ability to get ahead in technology will be based more on the capricious nature of my neighboring civs than any planning I do.
Also, tech trading had a really nice and consistent effect of keeping the technologies of all the civiliations in play close to each other. A few civilizations would get ahead because they were powerhouses while one or two others would lag behind because they were isolated and living on rock piles. 80% of the other civs could just trade with each other and the tech leaders to be able to keep up in technology even if they weren't necessarily power houses. In this way, civilizations including my civilization wouldn't be left in the dust simply because one or two civilizations got an early advantage, spent time teching, and used that technology to ****** other civs and exponentially grow their own teching potential.
If civs can't cooperatively tech easily even if they aren't on the best of terms, then I fear Civ 5 will be unbalanced. The comparetive tech of civs at the mid to later stages of the game will be completely out of wack where a few civs will have nuclear weapons where most other civs will be in the dark ages. Also, the civs that win will be those who have a slight early game advantage and are able to exponentially grow their technology.