I'm not an addon developer but as WoWAce "donated" (if you will) LibStub for everyone, which others are using. Maybe WoWAce use LibExtraTip, which, as per the link above, was developed not just for Auctioneer but for the WoW Community in general. (Which was the same for LibStub if I am not mistaken.)
I don't know, as I've said I don't develop addons, but maybe using the same "tooltip" Lib or working together for one-for-all tooltip Lib will be better. (I understand that it is hard to swing it to a particular development road :p )
Anyway.. back to IRL stuff... must finish before WotLK launch ;)
Completely different animal. In fact, it was the initial reason I wanted to rename LibTooltip - that used to be LibExtraTip's name and I wanted to avoid a namespace clash.
For the most part, LibExtraTip is exactly that - an extra tooltip in addition to GameTooltip. No major difference in functionality that I could see from looking at the code - though I did mostly skim. :)
LibTooltip, on the other hand, supports multiple concurrent tooltips with more than two columns as well as cell providers which in theory could support mouseclicks. We are going to rename LibTooltip soon (probably LibQTip), and push a release version.