Seagate recently released a 2.5" 160GB HD. My MacBook Pro has a 100GB and it is hardly enough to handle my "mobile" data. OWC has the seagate drive for sale for $289, w/ a free EMC Retrospect backup solution. OWC will also give you $60 for your used original 100GB Apple HD.
If you don't feel like doing the upgrade yourself, TechRestore offers an overnight upgrade service, where they will install the Seagate 160GB HD on your MacBook Pro, transfer the existing data to your new drive ensuring that the new drive is ready for use when the laptop is returned. Their price is a reasonable $329, the drawback is the 2+ days you will be without your computer.
Now, you can always get a portable firewire (or USB) case for your factory HD and use it for backup, or extra photo storage. OWC sells such a case for $54.99.
Interestingly enough, OWC sells the HD enclosure case pre-assembled with a 160GB HD for $299. You are better off buying this combo than buying the HD and Case individually.