Jon, The HDMI technology wroks great for stacking the PowerConnect 55xx series. You can only use the HDMI ports on the 55xx for stacking though and not for uplinking to a Cisco or other device.  Also you can only stack 55xx switches with other 55xx and running the same version of firmware.  On the 55xx, you don't need to reboot the switches, but just plug in the cables to create the stack.  An algorithm is used to determine which member becomes master, and wich members become slaves.  I believe the master is determined by which switch has been powered up the longest.  Just remember that if you have the switches in different subnets or vlans, they're configurations will now be changed since all switches will become as one large switch.  The type of HDMI cable is important, you will need to refer to the documentation that came with the switch, or you can download the User Guide at [URL]  You will need cables that provide throughput of 10.2 Gbps (max. allowed by HDMI specification).  Usually the cheaper cables do not provide this.   -Victor
We have 8 PowerConnect 5548 switches and are interested in using the HDMI stacking for 10GbE.  We currently have all 8 switches powered on and running production traffic on them.  Each switch is currently either "home run'd" back to a Cisco switch, or daisy-chained to another PowerConnect 5548 that is.  Thanks,Jon    
HDMI Stacking on PowerConnect 5548