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All Thankpads have a list in the BIOS of "acceptable" wireless cards. That list is called the "white list". The standard Lenovo BIOS permits only WiFi cards on that list, and the card must be Lenovo-branded, i.e., there must be an FRU# printed on the label of the wireless card.
If you prefer to use a card not on the "white list" or even a card that is listed but one which does not have a FRU# printed on it, then you must install a modified BIOS which removes the "white list" restrictions. That is simple, because there are multiple T400 modified BIOSs which will let you install any wireless card that will fit physically.
My recommendation is to install a modified BIOS, and then install an Intel 6300 wireless card if you have 3 WiFi antenna wires (gray+black+white) ..or.. install an Intel 6300 or 6200/6205 if you have only 2 WiFi antenna wires (gray+black).
The Lenovo T400 HMM (Hardware Maintenance Manual) has disassembly instructions and parts list:
http://support.lenovo.com/en_US/detail. ... MIGR-70069