Retail editions of Windows 7 Professional and Windows Server 2008 R2 can be converted to KMS clients, provided that the organization has acquired the appropriate volume licenses and conforms to the Product Use Rights. To convert Windows 7 Professional and all editions of Windows Server 2008 R2 from retail to a KMS client, skip the Product Key page during operating system installation.

(from elevated command prompt) slmgr -ipk <key>

Windows 7 Professional    FJ82H-XT6CR-J8D7P-XQJJ2-GPDD4
Windows 7 Professional N    MRPKT-YTG23-K7D7T-X2JMM-QY7MG
Windows 7 Enterprise    33PXH-7Y6KF-2VJC9-XBBR8-HVTHH
Windows 7 Enterprise N   YDRBP-3D83W-TY26F-D46B2-XCKRJ 
Windows 7 Enterprise E   C29WB-22CC8-VJ326-GHFJW-H9DH4 
Windows Server 2008 R2 HPC Edition   FKJQ8-TMCVP-FRMR7-4WR42-3JCD7 
Windows Server 2008 R2 Datacenter     74YFP-3QFB3-KQT8W-PMXWJ-7M648 
Windows Server 2008 R2 Enterprise  489J6-VHDMP-X63PK-3K798-CPX3Y 
Windows Server 2008 R2 for Itanium-Based Systems    GT63C-RJFQ3-4GMB6-BRFB9-CB83V 
Windows Server 2008 R2 Standard  YC6KT-GKW9T-YTKYR-T4X34-R7VHC 
Windows Web Server 2008 R2    6TPJF-RBVHG-WBW2R-86QPH-6RTM4

Windows Vista Business   YFKBB-PQJJV-G996G-VWGXY-2V3X8
Windows Vista Business N    HMBQG-8H2RH-C77VX-27R82-VMQBT 
Windows Vista Enterprise    VKK3X-68KWM-X2YGT-QR4M6-4BWMV 
Windows Vista Enterprise N    VTC42-BM838-43QHV-84HX6-XJXKV
Windows Server 2008  Datacenter    7M67G-PC374-GR742-YH8V4-TCBY3 
Windows Server 2008 Datacenter without Hyper-V    22XQ2-VRXRG-P8D42-K34TD-G3QQC 
Windows Server 2008 for Itanium-Based Systems     4DWFP-JF3DJ-B7DTH-78FJB-PDRHK 
Windows Server 2008 Enterprise    YQGMW-MPWTJ-34KDK-48M3W-X4Q6V 
Windows Server 2008 Enterprise without Hyper-V    39BXF-X8Q23-P2WWT-38T2F-G3FPG 
Windows Server 2008 Standard  TM24T-X9RMF-VWXK6-X8JC9-BFGM2 
Windows Server 2008 Standard without Hyper-V    W7VD6-7JFBR-RX26B-YKQ3Y-6FFFJ 
Windows Web Server 2008    WYR28-R7TFJ-3X2YQ-YCY4H-M249D

Details: http://technet.microsoft.com/en-us/library/dd772269.aspx
Deployment guide: http://technet.microsoft.com/en-us/library/cc303280.aspx (Vista/2008), http://technet.microsoft.com/en-us/library/dd772269.aspx (Windows 7/2008 R2)