Blizzard has amended this Bnet forum post with a short FAQ about the v1.10 patch. The newly-modified question is as follows:
Q: Will I have to get the new 1.10 Set Items to get the new bonuses?
A: Existing set items will not have the new 1.10 partial item bonuses, but will enjoy the new full set item bonuses. Only those set items discovered in 1.10 games will have the new partial set bonuses.
So basically you will get the full set bonuses with existing items, but must find new v1.10 set items to get the new hidden item/partial set bonuses. For example, take the projected v1.10 version of Trang-Oul's Avatar Set.
With a v1.09 Trang's equipped in a v1.10 game, you would get the full set bonuses, but you would not get the three +15% Mana Regen bonuses, or the ability to cast Fire Balls, Fire Walls, or Meteors. You would need to have equipped two v1.10 items to get the FireBall bonus, three for the FireWall bonus, and four for the Meteor bonus. The fifth item in the set could be the v1.09 verison you already have, so at least you don't need to find every set all over again. Of course if you play ladder chars, you'll be starting from scratch anyway.
This new info came about from an odd source, actually. Me. I was in the Bay Area last week, and was lucky enough to get an invite for a full day of play testing v1.10 at Blizzard North. While there I was able to talk at length with the v1.10 lead designer and he was able to explain the whole set item partial bonus thing that so many players kept asking about. And yes, I was able to play several hours of v1.10 with a variety of high level test characters, and yes, it's got a lot of great changes. Unfortunately for you guys, I signed an NDA and Blizzard has requested that I not release any new info about the patch... until the patch is released. I can't even give an ETA for when that might occur, but as we reported from E3, there will be a FanSite Chat in the next few weeks, and at that chat Blizzard will be releasing a lot of new v1.10 info, including an updated release date estimate. You can see some more info about my play testing in this forum post.