Details are short right now, but the university is citing "eligibility issues" as the official reason.
Coach Gundy's comments were just as cryptic, saying; "We're going to treat this as if those guys were injured and somebody else just needs to step up in their places. We have no way of commenting further until the issues are resolved. As soon as we get any information, we will share it."
I always hate to read between the coach/administration-speak lines, but the fact that the issues are unresolved, and there is no room to comment means that there doesn't seem to be much of an idea of when the issue will be resolved. Rumors of this being a full season of ineligibility are running rampant through Stillwater, but obviously that is as unverified as the A&M rumors (the SEC ones and the ones about the A&M grad that is objective, well-adjusted, and level-headed).
For the more casual fan out there, Thomas was slated to be the starting safety and will be replaced by the super-athletically-gifted, but inexperienced Daytawion Lowe. While Thomas isn't all-conference material (debatable), he had a very solid 2nd half of the 2010 season, and brought nice leadership to a secondary that isn't incredibly deep... and just got more shallow. Harrison has shown a lot of talent and promise, but his loss will not be felt as much since there is so much depth at WR.
Also of note, Thomas was slated to return kickoffs with Justin Gilbert. I assume he will be replaced by Josh Cooper or Joseph Randle. Randle returned 11 kickoffs last season, but now being the feature back they may want to protect him. Look for Colton Chelf as a dark horse candidate to be back there as well.
More details and rumors as we hear them.