[ESS] Noweb problem after upgrade to ESS 12.09

[r]

Hello, 
I installed ess latest version (ess-version : 12.09) a few days ago. I
work with Linux Ubuntu 10.04 (Lucid) and emacs 23.1.1. After the ess
upgrade I notice troubles working with noweb files (.Rnw)
In particular I have the following error: 

File mode specification error: (void-function noweb-mode)

This error compromise me to work with .rnw files.

Did you notice this kind of error before??
What should I to to solve it??

Regards
Riccardo