PC ==> USB ==> N900 ==> WIFI

YES. the n900 is almost there to get this job done, but nokia released it without several (2) pieces missing, so some customization is needed.

First, you need to connect your n900 as a usb network device
1) USB -> n900: enable usb networking:

then, you need to add to your n900 the capability to share the internet connection (wifi in this case)
2) USB -> N900 -> WIFI: enable n900 NAT, you need iptables and then route from usb to wlan0. you need to install Kernel Power (or to be smart enough) for the nat module, and iptables is available as a package.

from here, the connection throught n900 wifi is possible. you can do connections to an extranet ip, but they are not routed to the n900 automatically, for that
3) PC -> WIFI: You need to change the default gateway to the usb network interface in your PC. in linux, its a "route del default gw" (and readd the default gw with the new interface), in windows you need to hit your network settings with a hammer, it depends of your OS.

understand this not as a how-to, but as a guide

i connect the n900 to the pc in pc suite mode , i get the usb0 ( linux ) device in my ip , in the n900 side i run ifup usb0 that is get configure with the ip 192.168.2.15 , in the computer i configure the usb0 with 192.168.2.16 , from the n900 i can ping the pc but i can't ping the n900 from the pc. , anyidea ? what i am missing.