In other words, does Texas law already declare him to be a non-resident? Would he still be eligible for in-state tuition at UT, for example? I don't know about the former, I am quite sure that the answer to the latter is "no".
I'm not sure it really matters. Eligilibiliy or ineligibility to vote does not mean he is or is not eligible to run for office.
However, Texas law only appears to require that he be a resident on the day he votes. His current residency in Virginia and his voting in the primary there does not appear to disqualify him from returning his residency to Texas and voting there in November.
http://www.capitol.state.tx.us/statutes/docs/EL/content/htm/el.002.00.000011.00.htm#11.001.00
Here's a link to an opinion from the Texas Secretary of State's site that covers the issue of residency. It's specifically talking about college students, but if goes into what is required to establish residency for voting. I am not aware of a separate definition of residency relating to candidacy.
http://www.sos.state.tx.us/elections/elo/gsc1.pdf
There appears to be two requirements. A physical residence in the area in which the person wishes to be a resident, and the intention to make that place the person's home rather than a temporary residence.
The opinion quotes a specific case as follows:
As stated in the seminal case of Mills v. Bartlett, 377 S.W.2d 636, 637 (Tex.1964), “[n]either bodily presence alone nor intention alone will suffice to create the residence, but when the two coincide at that moment the residence is fixed and determined. There is no specific length of time for the bodily presence to continue.”
Delay is claiming that he is an ineligible candidate under the US Constitution rather than under Texas law.
Basically, if Delay chooses to make his home in Texas his home at 11:59 pm the night before the election with the intent to live there, he is eligible to vote in the election.
Since he is not required to be a resident before the actual day of the election, and there is no time period requirement to establish residency, it cannot be determined if his residency would disqualify him until the day of the election.
Otherwise someone could try and sue to have a candidate ruled ineligible even though they aren't required to be a resident prior to election day by the Constitution.
I don't know that the residency requirement are for in state tuition in Texas. In Ohio I had to be a state resident for an entire year before I qualified for in state tuition.