(via the very nicely done a href="http://www-clips.imag.fr/multicom/partenariat/Multifiche">french ergonomist website multifiches) Interesting comparison between the numerical keypad of a cell phone to the keypad of a computer: Rinck M. and Ellwart T. (2004). Repetition priming during the use and recall of numerical keypads. European Journal of Cognitive Psychology, 16, 841-861.

Abstract: In two experiments, we investigated how repetition priming--a type of implicit memory--may be used to improve recall of the layout of digits on push-button telephones and calculators--an explicit memory task. In Experiment 1, participants were able to use repetition priming by pretending to dial a well-known telephone number in order to recall the telephone layout of digits, improving encoding of the number and preparation of the corresponding motor program. In contrast, no facilitative effect of known numbers on encoding and preparation was found for the calculator layout. Experiment 2 showed that the priming advantage of the telephone layout is not due to a general inadequacy of the calculator layout: Experimentally induced semantic repetition priming and motor repetition priming were observed for both the telephone layout and the calculator layout.

It's about comparing the two different layouts. In other experiments it has been found that people memorize the phone numbers positions they know well. The authors validates this hypothesis and they found that this is not the case with computers. Another experiment shows that the this phenomenon is due to the fact that people often carry out the activity of composing known number on the phone keypad whereas they do not do it on the numeric kypad of the computer.