If a mass moves close to the speed of light, does it turn into a black hole? I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever since. 
From the show transcript: 

Arunus Gidgowdusk from Lithuania asks: "If you took a one kilogram mass and accelerated it close to the speed of light would it form into a black hole? Would it stay a black hole if you then decreased the speed?"

Dr. Gay, an astrophysicist and one of the hosts, explained that she'd asked a number of her colleagues and that none of them could provide a satisfactory answer. I asked her more recently on Facebook if anyone had come forward with one and she said they had not.  So I thought maybe this would be a good place to ask.
 A: The answer is no.
The simplest proof is just the principle of relativity: the laws of physics are the same in all reference frames. So you can look at that 1-kg mass in a reference frame that's moving along with it. In that frame, it's just the same 1-kg mass it always was; it's not a black hole.
A: I am presuming the idea is the 1kg mass will length contract to below the Planck length.  It is either that or the relativistic energy (mass) $E~=~\gamma mc^2$ would be so large it would gravitationally implode.  The question though can be thought of according to what would happen to an observer on the mass.  The question could be turned around: Would the universe implode?  If a mass $M$ passes by a smaller mass $m~<<~M$ then one might think that $M$ could become a black hole and the small mass $m$ if close enough would become trapped in the black hole.  However, from the frame of the big mass $M$ the small mass is not a black hole.  This is a contradiction.
A ultra-relativistic mass will behave similar to a gravity wave as it passes another reference point. This Aichelburg-Sexl ultraboost has a plane wave pulse of spacetime.  The relativistic mass will result in a gravity wave pulse as detected by a stationary observer.  So there is a gravitational implication to such extreme relativistic boosts.  
A: No, a 1kg mass would not turn into a black hole, even if it were zipping past you at very close to the speed of light.
The principle of relativity is a fundamental idea in physics, and one consequence of it is that we can understand the physics of something that's moving by imagining we're moving alongside it.  
For example, you are watching people play pool on a train as it rushes past you.  You want to know whether a certain shot that's just been made will sink the 8-ball.  You figure it out by imagining you're inside the train and calculating everything you'd expect to happen from that simpler viewpoint where the pool table is stationary.  If the 8-ball goes into a certain pocket from that point of view, you can rest assured it will go into the same pocket if you analyze the situation again from your original vantage point on terra firma.
Applying the same principle to the 1kg mass, we see that moving along side it, it just looks like a normal mass, not a black hole.  Hence, from another point of view in which it moves close to the speed of light, it still looks like a normal mass, not a black hole.
A: While good, I think the other answers are currently missing one ingredient, so I'll post this answer.
For particles traveling at constant velocity there is no event horizon, and so they act nothing like a black hole. Light from other regions of space will eventually reach it, unlike a black hole. Further, the forces between atoms in what ever matter constitutes the mass are co-moving and so there is no increased gravitational interaction between them. While the distances between them appear to change to an outside observer (as the mass is accelerated) once it reaches constant velocity they are fixed. 
What has not been mentioned in other answers is the effect of acceleration. When a particle is continuously accelerated there is an apparent event horizon. See the relevant Wikipedia page here. So this has some features that we associate with a black hole, however there are still major differences. An object undergoing constant acceleration does indeed behave like it is static in a constant gravitational field. However, in the case of such an object the direction of the equivalent field is constant (and in a constant direction) throughout the object. This is not true for the gravitational field of a black hole, which is spherically symmetric.
Of course once the particle stops accelerating the apparent horizon disappears.
