Happened to me yesterday morning. It was really really cold (45 degrees) and had been raining. Crank took maybe 2.5 seconds before it started. Brand new battery, too. I wouldn't worry too much about it unless it is the original battery on your 08.
2008 (07/08) E92 M3 BPM Stage II Tuned by Mike Benvo, 6MT, Turner Test Pipes, Magnaflow Custom Catback Exhaust (Magnaflow X Pipe, secondary cat delete, Aero Resonators and Helmholtz Chambers),Vorsteiner CF Lip, Hamann Style CF rear diffuser, CF Rear Decklid Spoiler, BMW Performance Kidney Grills, CF Side Grills, OEM GTS Wheels, K&N Drop-In Filter, GP Thunder 8k Angel Eyes, Painted Reflectors.