When playing with the raspberry Pi I noticed a couple of oddities. If I have a mouse and keyboard plugged into a usb hub the ethernet goes very slowly and as soon as X is run pretty much becomes unusable, filling the kernel messages with:

[ 2354.487491] smsc95xx 1-1.1:1.0: eth0: Failed to read register index 0x00000114
  
[ 2354.487530] smsc95xx 1-1.1:1.0: eth0: MII is busy in smsc95xx\_mdio\_read
  
[ 2359.487451] smsc95xx 1-1.1:1.0: eth0: Failed to read register index 0x00000114
  
[ 2359.487499] smsc95xx 1-1.1:1.0: eth0: MII is busy in smsc95xx\_mdio\_read
  
[ 2365.587487] smsc95xx 1-1.1:1.0: eth0: Failed to read register index 0x00000114
  
[ 2370.587541] smsc95xx 1-1.1:1.0: eth0: Failed to write register index 0x00000114
  
[ 2378.717626] smsc95xx 1-1.1:1.0: eth0: Failed to read register index 0x00000118

and:

[ 601.380138] smsc95xx 1-1.1:1.0: eth0: kevent 4 may have been dropped
  
[ 601.388141] smsc95xx 1-1.1:1.0: eth0: kevent 4 may have been dropped

This can can also be noted in the ping times to the local router, which are of the order of 0.9ms normally and 500-2500ms when a mouse and keyboard are plugged into the hub.

This seems to have been attributed to noise from somewhere, either from the hub or from using the HDMI (As the errors only appear to happen when X is running).

This has been discussed at https://github.com/raspberrypi/linux/issues/60

Temporarily worked around this issue by plugging both mouse and keyboard into the PI itself, although long term this is less than ideal.