try this again

Step 1 Turn off your phone
Step 2 Rub the metal part of the simcard before putting into the phone
Step3  turn on the phone

See whether it works or not.

Lastly mate do not ask for solution for unofficial firmware because its still bugged 

When i first got my nokia n900 i have such issue as well. Its just the device very sensitive to the particular simcard. So u can try taking out your digi simcard rub the metal part or use a rubber. Try it again

you cant reflash from PR1.2 to PR1.1.1 You will get an error receiving calls and sms cause the radio has been updated as well. there is a work around. Search on maemo.org

When i first got my nokia n900 i have such issue as well. Its just the device very sensitive to the particular simcard. So u can try taking out your digi simcard rub the metal part or use a rubber. Try it again

tested and it works. very buggy though.
you need to reflash eMMC/core with Vanilla firmware, only then reflash with PR1.1 again.

alternatively, reflashing back to PR1.2 will solve the sim card detection error instantly.

It only monitors GPRS,3G bandwidth

i play safe so its PR 1.1

From my understanding i wont touch the core of the phone i will touch what is outside the core

Install "HTOP", this app is pretty easy to use, just see which process is running at 100% all the time or high processor usage, select the process ( by touching or use the keyboard arrow keys), there's option at the bottom u can select an choose kill.
Make sure you know what you're using, some process are system process like hildon-desktop, etc. usually what I encounter is  browserd hongging the processes, even tough I  have no browser running.

The simple way is to restart the phone, if you aren't sure.
Yes, the leaked PR1.2 data counter has some changes, it added roaming counter, thus the current Personal Dataplan Monitor widget will not work.

errrmm... need some help here... my USSD widget is not working... it stopped working after i uninstalled the USSD pad. i really need USSD widget... =(

What video are you referring to?

I don't think the size matters so far, I've played over 3GB videos and I don't notice any difference during normal playback.

i'm learning how to hack wifi with n900. anyone has experience?

You might notice the battery drops faster from 96%/97% to around 80%++
It depends on your charge. Usually, if you left it charging after it's full, it actually stops charging (cut off).
So, if you leave it and later when you unplugged the charge, it will still show 96%/97%, but it usually drops very fast down to around 80%++. This I believe is because the battery actually already discharged during the cut off.
It won't recharge until your battery drops below certain percentage, I believe should be 85% (safety feature).
But if you let it charge full and unplugged immediately, it can retain longer.
Or you can try charging it when the battery below 85%, when it's full, try unplug the charger and see the difference.

Another thing is try turn off the other 3 desktop and leave your main desktop less widget, to preserve more battery.
For the battery widget, you can set it's refresh interval to longer/etc so that it don't use too much power.
If you have "System Info Widget", check and make sure the processor and ram usage is normal.

natively not supported. then those add on caused some problem to the phone i think..