This is my first guide, so please don't flame. If you disagree with something, feel free to say so. This is an ongoing guide, and will be regularly updated with any good feedback I might get, plus anything I might pick up as I use it more. Lots of information here is taken from various websites, as far as I can tell there are no copyrights to this information. Should you have any information here which is copyrighted, please let me know and I'll remove it.
DAVE'S SENTRY GUN GUIDE:
Contents:1: General information.
2: Statistics
3: Things to remember.
4: Defending the Sentry Gun.
5: Placement.
6: Unanswered questions.
1: General Information:The Sentry Gun in Call of Duty: Black Ops is a 6 killstreak reward, as opposed to a 5 killstreak reward in Modern Warfare 2. With Hardline it becomes a 5 killstreak.
It is still airdropped by a helicopter which can be shot down. When that helicopter gets shot down before it drops the crate, the crate will fall at the point of impact.
The Sentry Gun costs 3200 COD points to purchase. Once purchased, it is always available until you activate Prestige Mode, in which case it needs to be purchased again.
The Sentry Gun uses thermal to detect enemy targets, the only way to stay undetected by one is to use Ghost Pro.
There is no evidence that a Sentry Gun will shoot at enemy air support if placed high up. (If anyone can confirm the contrary, please do)
2: Statistics (Credit to Denkirson for these stats):

Health: 1000 health.
Duration: 90 Seconds.
Armor: 0.3x bullet damage (The armor reduces bullet damage taken by 3, per bullet)
Stun Duration: 5 seconds.
Damage per bullet: 20
Fire time: 0.05 seconds.
Delay between Salvos: 1 second (it also takes the sentry one second to fire after targeting an enemy)
Rounds per minute: 6000rpm.
3: Things to remember:3.1: The Sentry Gun is noticably more powerful than the one in Modern Warfare 2.
3.2: Once the Sentry gun is placed, it can only be moved by it's owner.
3.3: The Sentry Gun can be destroyed by one melee hit.
3.4: The Sentry Gun will not fire at RC-XDs.
3.5: The Sentry Gun can be hacked by Hacker Pro users. Doing so inflicts damage to the turret, and reduces it's lifespan. Once hacked, the turret will fire for the enemy team, and can no longer be moved.
3.6: The Sentry Gun will not disappear if you're killed.
3.6 When firing, the Sentry Gun will appear on the enemy's minimap.
3.7: Sentry Guns can not be taken out by team mates (can someone check if this is the case in hardcore please?)
3.8: The Sentry Gun is disabled for 5 seconds if hit by a stun or flash grenade. Nova gas has no effect.
3.9: Sentry Guns don't have infinite range. Placing one too far away from a busy route won't net you any kills, unless someone happens to walk by.
3.10: The Sentry Gun will shoot enemy dogs if they come in range. (Kudos to Lepercolony)
Defending the Sentry Gun:The Sentry Gun is very vulnerable to melee attacks. If someone manages to get behind it, they can take it out with one melee attack. Defending the Sentry Gun is important, as it's an important target for Ghost users. Placing the Sentry Gun in such a position that it can cover an entire side is beneficial. This will leave the other side vulnerable, but you can always place a Claymore Mine nearby or keep an eye on it yourself and shoot anyone that comes nearby.
Placing the Sentry Gun:The placing of the Sentry Gun is very important for two reasons. One is that you want to get as many kills as possible with it, and the second is that you don't want it to be too accessible for enemy players.
If anyone would like to add any map-specific placing ideas, please feel free to do so.
These are my thoughts on placing the Turret in general:
Always get as much field of vision as possible, without comprimising it's security. Placing a turret in the middle of an open space might seem like it will get you lots of kills, but odds are it will be taken out within minutes of being placed.
Placing it in a building might seem like a good idea, but windows can severely restrict it's line of sight. I personally find it best to place Sentry Guns on gantries, rooves, and corners where they're more likely to kill someone that's approaching them.
Something to keep in mind is that many players use the Ghost perk, which upgrades to Pro once a certain amount of challenges are complete. One of these challenges involves destroying a Sentry Gun. This can be used to your advantage, for example:
In Radiation, there's a little hut in the middle of the map next to the cargo doors, on the silo side. It has one access ladder going up to it. If you have a Ghost user or two on the enemy team, plave the Turret on top of that, facing the ladder. They'll want to take out the sentry, thus climbing the ladder to get to it. Once they reach the top, the Sentry will shoot them before they even touch base.
Unanswered questions:These are questions that popped up while writing this guide. I couldn't find any conclusive answers on the internet, and haven't had time to try it out myself yet.
1: Which air support is capable of taking out a sentry gun? Napalm? Carpet bomber?
If you can provide any answers, that'd be great.
As I said before, this is my first guide. If you'd like to contribute anything, please do so. If this gets a good response, I might do another one on SAM Turrets.
Thanks for reading this, I hope you enjoyed it / learned something!