My idea on the GUI was to develop at first a communication mechanism/layer, providing both JMX and Thrift functionalities, in order to achieve monitoring and querying capabilities like most DB web UIs do.
It would be good to know which features are most desirable, to develop a roadmap. Please comment any opinion/idea on this matter!
As for the language, personally, I don't mind if it's PHP or Python, BUT: I care about running the project in a language which is familiar to the Cassandra community. I wouldn't like to build a PHP UI which seems like a foreign member standing outside the Cassandra umbrella.
Nowadays, with modern browser is easy to have graphs updated on the run with any need of refresh. Take a look at YUI Charts polling mechanisms: