In fact, as Barr's website reports, both McCain and Obama failed to file the necessary paperwork by the August 26 deadline to get their names on the Texas ballot. But Obama wasn't counting on winning Texas, and McCain most certainly was. Of course, there will probably be some state-level shenanigans which end with the law being twisted (or broken) to put McCain on the ballot, but what does it say about John McCain that he didn't even care enough to get his name put on in a legal, timely way?
Texas Election Code § 192.031 permits a political party is to have their presidential and vice presidential candidates on the Texas general election ballot if "the names of the party's nominees for president and vice-president" are submitted before "5 p.m. of the 70th day before" the presidential election - that 70th day before having now passed over a day and a half ago. But, according to the Texas elections website (linked above), only one candidate met that deadline -- Bob Barr.
And Texas has 34 electoral votes, votes McCain was absolutely counting on to put him in contention in this election. Texas is more electoral votes than Pennsylvania and Indiana combined. More than Florida plus Nevada. Texas is New Mexico, Colorado, Montana, Minnesota, and Iowa lumped together. And McCain has just blown it. Carelessly. Stupidly. Incompetently.
Now, before we go celebrating, there's little doubt that McCain (and Obama) will end up on the general election ballot, although not by legal means. Laws will be bent or broken; lies will be told about who submitted what and when they did it. Assurances will be made that some functionary or other forgot to log some papers -- and, since litigation may result and questions may be asked under oath, it is likely that perjury will be committed. But this is the McCain campaign we're talking about, so no one will be surprised or even bat an eye when obviously perjured testimony is accepted by a Texas judge as good enough to put McCain on the ballot.
Nevertheless, there's sweet irony here. McCain has recently sued to kick Barr off the Pennsylvania ballot over formalities which are a lot less clear than the stone-set Texas deadline. But McCain's suit comes after McCain in 2000 had sworn never to try to knock an oppenent off the ballot. It will certainly be fun to watch McCain squirm out of exclusion from the Texas ballot while maintaining that Barr should be barred from the smaller prize of Pennsylvania!
Addendum
A number of commenters are calling this "debunked" not because the law is other than what it is, but because Texas has already decided to break its laws and put candidates on the ballot who did not meet the deadline. The excuse is that the conventions were held late (and apparently the parties are assumed to have no control over when their conventions are held). Also, it is said that some functionary or other forgot to log some papers. Or didn't I mention above that this lie would be told?