will t-mobile charge me for a tethering fee if i still use mobile hotspot or joiku app or even just the connecting my n900 with a data cable on my computer?

i know that they won't notice what device i'm using unless the product and its settings is fully customized under their network but since i'm on a family plan, i just want to make sure that no additional charge will appear on my mom's bill...

I have also wondered this for a long time because i usually tether from my phone when im on the road.

I've not seen any additional fees for tethering my laptop from any phone. I'm on the $30.00 "no overage fee", formerly known as the "unlimited", plan. I imagine someone held their feet to the fire about it being unlimited when it was actually limited in speed after the first 5 GB in a month.

No, T-Mob do not need any notice for tethering a laptop (or whatever) to your device.

You will still be using the same internet data connection as you would if you were just browsing on you N900. However, be sure that you are the correct data plan for your needs as you may find that the charges can be very high if you exceed the date limit.