Wie TobStar schon richtig sagte: Koffein behindert die Creatinaufnahme nicht. Es wird aber diskutiert, dass chronischer Koffeinkonsum möglicherweise den kraftsteigernden Effekt von Creatin beeinträchtigen kann (allerdings gibt es zu dieser Frage herzlich wenige Studien). Der genaue Wirkungsmechanismus hierfür ist aber noch nicht vollständig geklärt.
Ich glaube aber nicht wirklich, dass die tägliche morgendliche Tasse Kaffee die Creatinwirkung signifikant beeinträchtigt. Im Zweifel sollte man aber vielleicht den Kaffeeverbrauch auf geringe Mengen einschränken.

Die bekannte Vandenberghe-Studie:
J Appl Physiol. 1996 Feb;80(2):452-7.

Caffeine counteracts the ergogenic action of muscle creatine loading.

Vandenberghe K, Gillis N, Van Leemputte M, Van Hecke P, Vanstapel F, Hespel P.

Faculty of Physical Education and Physiotherapy, Department of Kinesiology, Katholieke Universiteit Leuven, Belgium.

This study aimed to compare the effects of oral creatine (Cr) supplementation with creatine supplementation in combination with caffeine (Cr+C) on muscle phosphocreatine (PCr) level and performance in healthy male volunteers (n = 9). Before and after 6 days of placebo, Cr (0.5 g x kg-1 x day-1), or Cr (0.5 g x kg-1 x day-1) + C (5 mg x kg-1 x day-1) supplementation, 31P-nuclear magnetic resonance spectroscopy of the gastrocnemius muscle and a maximal intermittent exercise fatigue test of the knee extensors on an isokinetic dynamometer were performed. The exercise consisted of three consecutive maximal isometric contractions and three interval series of 90, 80, and 50 maximal voluntary contractions performed with a rest interval of 2 min between the series. Muscle ATP concentration remained constant over the three experimental conditions. Cr and Cr+C increased (P < 0.05) muscle PCr concentration by 4-6%. Dynamic torque production, however, was increased by 10-23% (P < 0.05) by Cr but was not changed by Cr+C. Torque improvement during Cr was most prominent immediately after the 2-min rest between the exercise bouts. The data show that Cr supplementation elevates muscle PCr concentration and markedly improves performance during intense intermittent exercise. This ergogenic effect, however, is completely eliminated by caffeine intake.
Und eine mögliche Erklärung des Koffeineffektes findet sich hier:

Opposite actions of caffeine and creatine on muscle relaxation time
-> http://jap.physiology.org/cgi/reprint/92/2/513.pdf

Gruß