cameraiphone
I don't think I have ever used the digital zoom feature in the Camera app. The controls are just in the way when you wan't to focus something in that part of the image. So, if possible, I want to hide them. I'm not afraid to jailbreak my phone if that is what i takes. Any answer appreciated.
I have an iPhone 3GS.
This is, unfortunately, expected behavior. Whenever you activate an iPhone (which you do as part of an OS install) it talks to Apple's servers and does various things like exchange crypto certs for APNS, etc. It also checks the IMEI of the phone to see what carrier Apple sold it to, and validates the SIM is valid for that carrier (if you buy an unlocked iPhone in a country where you can buy either locked or unlocked phones the vendor will actually copy down the IMEI number of a normal locked phone, and then enter in their system where it gets pushed to Apple's servers as an unlocked phone, there is no physical or configuration difference on the phones).
The easiest way to get it activated is (as you surmised) to use an AT&T SIM. You mentioned you have an iPhone 4, you can activate it with that SIM. There are plenty of companies that sell microSIM to SIM adapters, but if you want to do it right now you can just place the micro SIM directly in the iPhones 3GS's SIM try with tape. I would not recommend doing this on a device you intend to keep the SIM in for a an extended period because the SIM is likely to come loose (and jam the SIM tray), but if you are just inserting it, activating it, and removing it you should be fine so long as you are pretty careful with how you apply the tape and not shaking the phone too much.
Since I'm just a generic curious who can do some research on the web, I'll make here some assumptions so we can be on the same grounds:
If you follow every link above deep enough, you may realize it's just not that simple. Even Android's TRIM support probably isn't the same TRIM as a notebook uses on its SSD.
So, it doesn't matter if it supports TRIM or not. The whole point of going with an iPhone is trusting Apple and letting them do the hardcore tech work. And the whole point of jailbreaking is being able to do things Apple can't allow for any reason, such as unblocking carriers. Anything else on JB is doing it wrong (specially now we have android, even for iPhone) - and I'm saying that from my heavy experience using jailbreak from iPhone 3 to 4.
Now, to address your main and unannounced question, best thing you can do to test if your hardware is malfunctioning is to backup your iPhone and reset to factory. Then fill it up with music or pictures, since you suspect a full disk is causing the malfunctioning, and avoid installing applications at all. You probably won't see any problem and, if you do, just send it to warranty. You can also send it to warranty directly, but you risk of it coming back and just losing few days there.
Best Answer
Currently there is no officially supported method of disabling the digital zoom feature. One way around it that I use when I (rarely) need to focus on the lower screen portion, is to focus on the subject, then move the camera's view to the preferred state. The camera will preserve this focus state long enough for a picture.