Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of third-largest American mobile communications provider Sprint Nextel (NYSE: S) are singing the blues today, falling as much as 17.4% on truly epic trading volume.

So what: AT&T (NYSE: T) and Deutsche Telekom unit T-Mobile USA just announced plans to merge, thus removing the rumored option of Sprint's merging with T-Mobile. Moreover, this deal would recast the competitive landscape in a whole new mold, where Sprint would become the largest mouse running around the feet of two enormous elephants.

Now what: If Verizon (NYSE: VZ) wants to counter the AT&T offensive by buying Sprint, the company just became about a billion dollars cheaper and more inviting for a Big Red proposal. But that's a big "if," and the AT&T deal that triggered today's action might not pass regulatory scrutiny to begin with. The mobile market just got a lot more complicated, and there are no safe bets here.

Interested in more info on Sprint Nextel? Add it to your watchlist.