Edit: I've made a mistake. Please accept my apologies for giving you the wrong information. Sjgadsby is absolutely correct. I wrongly said the wifi antenna was the bottom L shaped one - it's not, that's the Cellular antenna. Sorry. (Brain fart)

The WiFi / Bluetooth antenna is at the opposite end - the plastic looking thing above the camera.

The actual WiFi chip itself is obviously on the motherboard, but it's located on the keyboard half not far from the front facing camera - just a little to the right.

The position of the chip itself is not terribly important - however for a good description of the internal organs find yourself a copy of the L1 - L2 service manual.

Nokia_N900_RX-51_Service_Manual_L1L2_v1_0.pdf

The L3 - L4 manual more clearly shows the position, but this document isn't so easy to find.

Again, sorry, I messed up.