While many see the appearance as confirmation that Undertaker will be competing at WrestleMania, that is not necessarily the case. A WWE source told me that he believes that Undertaker was "testing the waters" to see if he could actually work a quality match at WrestleMania. I haven't heard any word yet on what the consensus was following the match, but the source said that he expects Undertaker to work the event.
PWInsider reported last October that the hip replacement Undertaker had done several years ago was badly damaged during his match with Triple H at last year's WrestleMania. He was so beat up last fall that he stopped seeing specialists in New York, and WWE flew doctors to his home in Texas. The Wrestling Observer reported that Undertaker had major shoulder and hip surgery over the past year, which made it impossible for him to do any serious training for some time. He reportedly couldn't work his upper body because of shoulder rehab, and couldn't do any lower body because of the hip rehab.
The Wrestling Observer also recently reported that Undertaker didn't want to return and have a mediocre match, and only wanted to come back if he could give a good performance.
I personally wouldn't be surprised to see Undertaker work more live events to get back into ring shape if he does end up competing at WrestleMania.