wow, this is gonna be tough.....
So I spent the xtra $$$ for next day delivery of Xenology ^-^, and was thumbing through it, and found a pretty solid hint about a Tau origins/Eldar connection. Now this post is just to encourage you to go get the book and look for yourself, not a giveaway or spoiler... If you want details, then priv. msg me!
Anyways, lets see.... Ethereals came out of no where, you know, at a certain measurable amount of time ago... at this very same time, the Eldar were up to no good in their mysterious ways: coming out of nowhere and bargaining with another Race with a Service, but when it came time to pay up, the Race in question wasnt willing to pay the price... and then the Eldar STOLE their 'price' nevertheless!
This 'price' and the Tau are very very closely related, and its something the Eldar themselves would have had no use for, but may have been a tool to steer their as yet unknown plans for the Tau in a favorable direction.... hint hint.. as to the WHY... well, its left unsaid... talk to the Eldar...
The Dates match up, the Price matches up, the.. geography makes sense... etc. Nothing was actually SAID in the book, but, if you read what I just read, and saw the pics I just saw (pics do tell a thousand words... even maps...) you cant miss it! and please, no 'give away the answer' posts... :

heh I guess theres no way to talk about this then...