Incorrect - Gmail is offering up the cell phone option in order to increase account protection. It is NOT mandatory.

From Gmail's Security section:
We all may forget our passwords at some point, so we strongly encourage that you update your account recovery options. To update these options, sign in to your Google Account by going to [link to www.google.com (secure)] and click Change password recovery options.

* Recovery email address: We can use your recovery email address to communicate with you if you lose access to your account. Learn more.
* SMS: We can send you a recovery code to your mobile phone number, which you can use to reset your password. Learn more.
* Secret question: This option is only available if you can't use the above recovery options and only if haven't tried to sign in during the past 24 hours. An ideal answer to your security question is easy for you to remember, but hard for others to guess.

The bold section is why they ask for the phone number.

Google voice users - you should not use this number for the recovery. If your email account is compromised, so is your google voice.

Watch for more changes/options in the near future.