First West of England are piloting a Smartcard for use on their buses. It's called '
touch'.

Initially covering just the
Bristol Inner and Outer Zones and the only products currently available to load on it are Adult Weekly or Monthly Season Tickets.
After a trial period the Smartcards will become available for Children, Students and Young Persons. The area of validity and ticket types availalbe to load may also grow after this pilot scheme has been assessed.
'
touch' cards can be obtained online
here, or from the First Travel Shops at Bristol and Bath Bus Stations.
Loading of products is only available on buses, or in the First Travel Shops at Bristol and Bath Bus Stations (despite Bath being outside the trial area

) at the moment. Online top-up is under development.
No mention yet of a Pay As You Go option.
More details at:
http://www.firstgroup.com/ukbus/touchsmartcard/