This is the exact way I screwd up my N900. My first N900 died because the USB port broke, the second because the GSM chip died. I tryed heating up the chip, using a soldering iron, and I probably f**ked it up more, or I burned out the chip.

Try the spring method (but be careful while removing the metal shield. You will have to put it back in one place and keep it tight). Before this try paper over the sim, to keep it in place. Then the glue method.

This will prolong the life of your N900 but not for long. I am sorry. I am now searching for a local service that can do fine soldering. I have a motherboard with good GSM chip, so I think I can try to replace the chips, but for now. My N900 is just a tablet pc, so useless for me.