It is, yeah. He can only regenerate 12 times, meaning as soon as we're up to 13, it's over. Supposedly. However, I quote Wikipedia:

The serial The Deadly Assassin established that a Time Lord can regenerate twelve times, for a total of thirteen incarnations (although at least one Time Lord, The Master, has managed to circumvent this.)

So, don't fret. Logic will not keep them from continuing the show xD.