I have a 2.5" SSD (Crucial CT480M500SSD1) that I want to use as my boot drive on my mid-2010 8-core Mac Pro. It doesn't fit the hard drive mounting brackets, which are designed for 3.5" drives. Because the SSD (a) is very light and (b) has no moving parts (and therefore no gyroscopic effects and no vibration), I'm considering mounting it without using a bracket at all—just plugging in straight into the socket, and letting the socket hold it in place. Is this reasonably safe?
If you plan on "plugging in straight into the socket" that would be a very bad idea. The SSD is light, but all of that weight will now be supported entirely by the two SATA sockets on the computer and drive. Over time the small amount of weight on the SSD will brake the bracket on the Mac Pro (expensive), or worse the drive (data loss).
If you can find the correct cable you could run a cable from the socket to the hard drive, and double stick tape the drive like the above answer said.
Or just buy a 3.5 to 2.5 adapter and install it in the drive carrier. It is cheaper than repair.
In my opinion it is reasonably safe. I would do it under your circumstances. However, my caveat to this is if you move your computer at all do something to secure the drive.
One more thing to consider: using some double-sided tape to stick the drive to the mounting area. I did this in my iMac when I added an SSD for a home-built Fusion drive.