It's the same bug as Bug 9 for 2100 driver. We retested it on Dell D505,Fedora 
Core 2,2.6.7 kernel for V0.8 2200 driver,and found the bug again. Don't know 
if our 2200 driver won't care about this feature at all.
So,following description is from Bug9, which was submitted by Julie:

Feature request to add the functionality to turn rts off 
or set it to automatic. Impact to user is if rts is turned 
on, it cannot be turned off without reinstalling the driver.

To reproduce:
# iwconfig eth1 rts off
Gives error:
Error for wireless request "Set RTS Threshold" (8B22) :
SET failed on device eth1 ; Invalid argument.

The same error is received for:
# iwconfig eth1 rts auto

Note that configuring a valid rts value (like 300) works 
as well as 'iwconfig eth1 rts fixed' (although I don't 
know what 'fixed' sets rts to).