it is possible to do in CUCM 6.1 so it,s no reason why can no be possible i n CUCM 7. I solve this isue with creating new entry for  end user and for cell phone number without Device Associations.

User ID XXXXXXXXXX

last name Y

First name X

Telephone number - 7568

Controled Device (select IP phone)

Primary Extension 7568

For cell phone

User ID XXXXXXXXXX - cell phone

Last name Y

First name X - Cell phone

Telephone number - cell phone

Controled Device - none

Primary Extension - none

In this way when you search corporate directory for user X you will have two resaults

1. First name, last name, primary extesnsion

2. First name - cell phone, last name , cell phone number