Customize the Boot Camp Drive Icon in Lion
Will Candybar work for you? It allows you to change icons, but I'm not sure about drive icons. Lemme know if it works for you.

Ok, according to this article, you can:


Paste your icon (in the Get Info window) onto a USB thumb drive formatted as FAT32.
The name of the drive doesn't matter.  
Launch Windows through Boot Camp, Parallels Desktop, or VMware Fusion.  
Open the thumb drive in Windows.  
Select Folder Options… from the Tools menu, and set it to show invisible files.  
Copy the two files .VolumeIcon.icns and ._[cr]File, where [cr] is a carriage return, to the NTFS drive.  


According to the article, that'll work.