Here you got a good explaination of how the counters work. 

Video : http://blip.tv/datastax/counters-in-cassandra-5497678 
Sildes : http://www.datastax.com/wp-content/uploads/2011/07/cassandra_sf_counters.pdf

I think it will answer your question and help you understanding interesting things about counters.

If you need a short answer, it concurrency will be handled nicely and you'll have the expected result.

Alain

2012/3/14 Tamar Fraenkel <tamar@tok-media.com>
Hi!

My apologies for the naive question, but I am new to Cassandra and even more to counter columns.

If I have a counter column, what happens if two threads are trying to increment it's value at the same time for the same key?

Thanks,

Tamar Fraenkel 
Senior Software Engineer, TOK Media