The specific rule is in the ITTF Handbook for 2005 (available at www.ittf.com). It's not in Section 2 - The Laws of Table Tennis, so most people who just have the basic rules wouldn't know about it. The rule is in Section 3 and is:
3.2.1.3 The covering material on a side of the blade used for striking the ball shall be of a brand and type currently authorized by the ITTF and shall be attached to the blade so that the brand and the ITTF logo are clearly visible near the edge of the striking surface.
Also, as mentioned on page 8 of ITTF Technical Leaflet T4 - Racket Coverings:
The supplier and brand names and the logo must be visible on the covering when it is mounted on the blade (see The Law 3.2.1.2).
Lists of all approved and authorized equipment and materials are maintained by the ITTF Office and details are available on the ITTF web site.
Now, some people will argue that because this rule is in Section 3 of the Handbook - which applies to International competitions - it shouldn't apply to small local tournaments etc. But the ITTF have covered this in rules of Section 3 as well - making any open tournament covered by this rule - see below:
3.1.1.3 An open tournament is one that is open to the players of all Associations.
3.1.2.3 The Regulations for International Competitions shall apply to
<snip>
3.1.2.3.4 open tournaments, except as provided in 3.1.2.4.
3.1.2.4 Where an open tournament does not comply with any of these regulations the nature and extent of the variation shall be specified in the entry form; completion and submission of an entry form shall be regarded as signifying acceptance of the conditions of the competition, including such variations.
So basically, unless a tournament entry form says otherwise, the rubbers should have the name and logo clearly visible. This will probably apply to most pennant or league competitions too, as most of these usually adopt the standard ITTF rules.
