if it works, you may try this:
1. insert microsd card (preferably clean and formatted) to microsd slot (it should be big enought to hold all data from your internal n900 storage, so 32gb will be big enought for sure, but if you have less data - maybe 16gb or 8gb will do)
2. power on n900
3. press these three keys at once: "Ctrl", "Shift", "X" - this is a shortcut to open x-terminal with path set to /home/user
4. type (be carefull, as you cannot see what you're typing and better not make a typo, note this is case-sensitive): 5. wait a few minutes (maybe even longer... i don't know how much data you have on your n900, but i guess copying all files on-device is quite time-consuming).

If you remember your folder names, you can use them in step 4 - replace "MyDocs" with "MyDocs/FOLDERNAME", e.g. "MyDocs/Downloads"

Before you start, you can also do 1, 2 and 3 and try to type: Then remove the microsd card from device, put it in computer and check if there is file "text.txt" on it - if there isn't - there's no point in doing that as there may be more damage than just gpu chip. You may want to ask in some hardware-related threads here wheather the lock key is software- or hardware-handled. I mean, the device may be completely damaged, but one little chip (which may accidentaly be responsible for led backlight and lock key) may still work.

I hope you will get your data back

SIDE NOTE: well, CSSU-guys, maybe (I have no knowdlege of disk mounting scripts/software in maemo) you could change /media/mmc1 mounting point to something more like /media/sd1, as mmc cards are no longer used since n800? (correct me if i'm wrong)