Aaron first of all thanks for your precious help everytime ....

Some resources for CQL 3, it may match your needs. If not you can still use Thrift through your favourite client...
There have been a few articles on the DS blog http://www.datastax.com/dev/blog
A talk at the conference by Eric  http://www.datastax.com/events/cassandrasummit2012/presentations
I did a webinar about it last month http://www.datastax.com/resources/webinars/collegecredit

I will read all the links carefully. The idea was to keep Pelops (client I am familiar with) and include CQL3 through the new Java driver Datastax is going to provide.

SELECT * FROM ctable WHERE basekey = ?  and myvalue = ? ORDER BY extensionkey 
DESC LIMIT 5
I haven't been able to do it ….

That looks ok, what was the error ? 
What cassandra version and what CQL version?

Cassandra 1.2 beta2 and CQL 3 -- honestly I didn't remember the exact  error (only that it was about the order by) and I don't have Cassandra here to try. Will write more about this tomorrow ... however if I avoided the where on secondary indexed column the query was ok. 

2) Is Cassandra still schemaless? One thing I loved is that to create a new 
column I didn't have to "alter" any cf before. 
Trying CQL 3 I noticed that if I try to make an insert of a new "column" not 
defined in schema I raised an exception. 

CQL 3 requires a schema, however altering the schema is easier. And in 1.2 will support concurrent schema modifications. 
Thrift API is still schema less. 

What it means that it will support "concurrent schema modifications?" (if the answer is in the link above I will know tomorrow :) )
I imagined that only CQL required a schema. What happen in a situation like this?

1) I create a table using CQL
2) I add a new column using Thrift
3) I query for the column using CQL

One more question:
Is there any noticeable performance difference between thrift or CQL3?

Thanks,
Carlo