At 30m you would probably get away with raw serial especially if you ran it at 9600 Baud. I run one at 50m from another arduino using software serial on one of them. The other has an ethernet shield so I can transfer the data across town to my server (grandiose name for a hacked Seagate Dockstar NAS box
) which plots the graphs.
The 'server' webpage is at http://pluggy.is-a-geek.com/
- the church graphs near the bottom are what it gets across town, the different data is collected from 1 Arduino with etherent and another (actually a RBBB, not a full arduino) at the opposite side of the building over serial. I use CAT 5 and use a couple of the spare cores to power the remote unit down the same cable.
The church end data can be accessed directly at http://majestic81.plus.com/
The previous poster is correct : the Ethernet shield is slow, its faster than serial at 9600 but not as fast as at 115200 baud. (around 4k a second). The above church end link is 'appropriate' to its capabilities.....