[T-Mobile Samsung Galaxy S II SGH-T989] itsfredo:
Symptoms:

Extreme Case: Once phone is plugged in or battery inserted, phone vibrates constantly and never progresses further through the boot cycle.

Moderate Case: Phone is able to make it to Samsung logo, or sometimes even further, but never fails to reboot and remain stuck in this cycle.

Preliminary Case: Phone randomly reboots for no reason, or when power button is gently touched (not hard pressed)

Problem:

1) Either phone button is sticky for some reason (ewww) and gets stuck, causing phone to reboot.

2) Or, (more severe cases) the internal switch is beginning to malfunction (common) and needs to be replaced in order to really fix the problem.

Fixes:

1) If the button is sticky, simply clean it you sicko and quit being dirty. Plenty of guides on how to open the phone on Youtube. (This is not really a big issue though)

2) Faulty hardware is going to require some major work to be completely fixed.

Ghetto Fix:

Intro: My phone is showing preliminary issues of the common switch malfunction issue. I read around and it seems that Samsung just failed with that portion of the hardware. My brother's phone, on the other hand, was an extreme case and was basically a brick in my room for a few months until I decided to fix it. It's not a profound fix, so I'm sorry for those who really want this issue solved. If you do, I recommend either replacing the part yourself or taking it to a shop to repair it for you. This fix is for those of you who might have an fully functioning galaxy s2 laying around, with the exception of that stupid button.

1) You will need to know how to open up your phone. There are many guides on Youtube. Be very careful doing so.

2) Ghetto Button Fix (Two Options) *From here on I will have to make a distinction between the two senses of "power button" I am talking about: There is the power button on the logic board (LB Power Button [my term]) and the one on the outside of the phone---the (P Power Button [my term]) is the one you see and press when your phone is put together and working fine--the (LB Power Button) is the switch attached to the logic board

a. The first ghetto fix will work if your case is not too severe, and it will look less ghetto. When you open your phone and separate all the parts, you are left with a good view of the (P Power Button)--you will notice that a small rubbery rectangular portion of the power button--file that off completely to prevent the (P Power Button) from touching the logic board power button (LB Power Button).

*Note: This will not be a fix for more extreme cases because the (P Power Button) can still reach the (LB Power Button). This will only make it less likely for the two to make contact when you do not want it to. Also note that the issue is not with the two making contact, it is a faulty switch. Anything from a quick jolt or vibration, or even gravity can be enough to set it into a looped cycle.

b. If the first one doesn't work, then simply remove the power button (P Power Button) entirely. With my brother's phone, I simply removed the button and reversed it. This way, there is no way for the two to make contact, even when pressed. We gorilla taped it into position.

*Note: The issue with this fix was that the phone could not be turned off (unless you take off the tape and access the switch to turn it back on). I gave him my extended battery to ensure he would not run out, and Installed a custom Rom which allowed him to use the volume keys to wake his phone from sleep (this might come stock in previous versions of andoid--dunno though), and change the power key/lock screen key to long pressing the home button. That way there is no need for the power button as long as the phone is on. (We used Beanstalk Rom 4.3) for this and it has been working flawlessly. **Since installing custom roms and rooting only requires reboot, it is still possible with a phone with a power button issue.

3) How to stop the boot cycle: It might be quite obvious, but just to state it--in order to stop the reboot cycle and "unstuck" the power switch, simply press the power button down a few times patiently until it remains stable. **Pressing it harder does not increase your chances! Remember to be gentle because you are working with a switch that is already faulty!

*Note:When the power button is removed, you can use some non-metal pointy object to press down the power switch inside the phone. You want to do this carefully, as you will notice that even the slightest touch of the switch, even if you don't press the button down, will send it into loop. Once you get the phone on and stable. Seal up the hole and the phone should be fine as long as it doesn't shut off.

Forgive me, I don't know how to make tutorials. Tried to catch everything I ran into along the way, but I noticed it's quite wordy. Hope this helps someone!