Check and see if it's inside the battery compartment.  I believe Dell started doing that with some new laptops to keep the sticker from getting worn off. If you use the Dell Vista CD, you won't need to enter the key anyway.   The Dell disk is pre-coded to check for a valid Dell BIOS key; if it matches it will automatically activate. 
just bought a new studio17 laptop and put vista ultimate on it and to my horror dell have not included the vista number or the sticker under the laptop.. surely they would not have been so stupid to miss this out, what should i do i need the license, for 1 i love t refomat all the time and also isnt it breaking the law
no license on my new lap top