I have a dedicated RAID 1 (mirrored) volume for my photo library which is stored in Aperture 3 and one of the drives just died so I'm going to replace both. Is there a noticeable performance improvement using 7200 RPM drives over 5400 RPM drives for photo editing? I'm sure there is for video editing but don't know about photo.
Yes, there is a noticeable improvement if you're using a DAM tool like Lightroom or Aperture. The bottle neck in such programs is the disk drive. To see this for yourself, import a set of files and then watch the Activity Monitor. With Lightroom, you'll see that the disk activity will hit 100% while thumbnails get generated. CPU activity meanwhile will be nowhere near 100%, especially if you have a fairly modern machine.
I work with 21MP photos and in an effort to speed up Lightroom on my desktop I looked at what I could improve and it seemed replacing the disks was the way to go. Unfortunately, I can't say about 5400 RPM vs. 7200 RPM but I replaced a pair of fairly zippy Hitachi SATA 15,000 RPM (!) drives by a pair of 160GB Intel X25G2 SSD. The improvement was noticeable, so my gut feeling is that anything faster does help. I don't edit photos on my laptop too much but I've had laptops in the past that I upgraded to 7200 RPM for work (compilation of apps), and I could also feel the difference.
My answer: yes, I think it will help. My advice: go for a SSD, besides memory this is one of the "cheapest" way to really improve your performances (short of buying a new computer). You don't need a big one, as long as you leave the Aperture/Lightroom database on this disk, and say, 6 months worth of latest RAW. If you don't have much memory, putting the OS page disk on the SSD can help as well.
I'm going to speak from personal experience here and apparently contrary to popular opinion. I recently (within the last 3 months) switched from a 5400 rpm drive in my laptop to a 7200 rpm drive. While the difference in many things was quite significant - I didn't feel my photo editing experienced much of a bump up. I'm not saying that there wasn't a difference, it just wasn't very noticeable - for me. I suppose this could be highly dependent on your specific workflow though.
When I upgraded my laptop hdd from the slow drive apple shipped in it (4800?) to 7200 there was a noticable improvement in opening just about anything. Everything felt just a little bit zippier.
EDIT TO ADD: In hindsight, the huge difference was probably more because the old drive was dying and there were a lot of IO timeouts waiting for it.