I always edit on my desktop. However, I will need to start using a laptop for times when I have to go to a client's office or out in the field. Finding a 7200 RPM hard drive is not a problem with desktop computers, but when it comes to a laptop, they tend to be hard to find (especially since I may need to get one locally instead of online). How much am I going to be screwing myself if I buy a laptop with a 5400 rpm hard drive instead of a 7200?
To be honest I wouldn't even consider buying a laptop that didn't have at least a 7200RPM drive in it. If I had plans to edit with the laptop. Your systems overall performance will suffer notably, your application launch times will be slower etc... Your seek times will be slower the computer will just feel slow overall (generally speaking). With SSD prices being fairly reasonable now days I'd really recommend getting a SSD to use as your OS drive. Having a SSD to run your apps from really makes your system feel much more responsive.
You would also need to buy a external drive, ideally some type of RAID array. But at the very least a 7200rpm single disk that uses E-SATA, or at least USB 3.0.
Since I'd be going to a client's office, carrying a RAID array around with me would be kind of difficult, but I was planning on getting a large external drive anyway. Would I be able to get away with getting a laptop with a 5400 internal and a 7200 external for now? I'd set up all the scratch disk settings to go to the external drive. The reason I ask is I may have to start working Monday or Tuesday for this client so I don't have time to order one online, especially if it's a special build. I may have to go to Best Buy (ugh) for it.
You can get away with it, you really just need to be sure though that the external disk you use is not only 7200rpm but also is using E-SATA or USB 3.0. They do sell raid-0 arrays that are fairly small some of them are the same size as single external disk. Although I realize you won't be able to get that by Monday but it would be a good long term option. Also make sure the laptop has a 64-bit OS and has at least 8Gb of RAM.
Long term though if the laptop is going to be your primary edit machine I'd really recommend upgrading the internal disk at some point down the road.
*Edit I was typing this while John was typing his answer (Didn't mean to post duplicate info)
.....in a new laptop, you can take good advantage of the SATA III speeds of the hard drive bays and put SSDs in them....use a marvell controlled SSD for the second drive, not sandforce, in order to have a drive which handles " incompressable" video files well at top speed, like a crucial m4, for cheaper now.....best ones are plextor pro series and Corsair pro series SSDs. Combined with an NVidia 6?? Series video card and at least 16GB memory,you can fly....check out Asus models,too.