I've had the exact opposite experience with the iPad/iPhone issue for a couple reasons...
1) Apps usually cost more for the same thing.
2) Lots of apps aren't universal. You'll have to pay for the same app twice if you want it on both devices. Yes, you could upscale it, but I really don't like the looks of it.
3) No camera. I constantly use this to create flashcards and scan my notes using apps such as Cam Scanner. Also, it's just easier to handle on my iPhone for these tasks.
4) Mobility, I usually don't bring an iPad with me everywhere I go. I have real work to do, so I bring my MBP. MBP + iPad + iPhone is just a little over kill I think. Also, I can't really whip out Plants vs Zombies while waiting for my ride to get there, its just awkward imo. Much easier on the iPhone.
5) 3G, if you already have an iPhone, its hard to justify paying for it (i know its optional) when you already have another 3G device in your pocket. Do you really need two? It may be convenient, but if i'm out and about, 3G is used to quickly check something. Weather, facebook, directions, etc. All of which the iPhone do just as well. On vacations, if i'm web browsing, i'm doing something wrong. You should be out having fun!
6) Really varies from person to person, but I just find it much more comfortable playing games on my iPhone.
7) Reading...call me old school, but I still use the hard copies. I just find it a tad heavy, also the backlit screen makes it harder for me to go to bed when i'm ready.
8) Music. I see people listening to music on this thing and they have an iPhone next to them...its way more convenient to have a pocketable music player.
9) Email, everyone always said it was so great to reply to emails on an iPad. It really isn't all that special...You can still do it from your iPhone! If you have to type a long email, the touch screen sucks to type on anyways. Bluetooth keyboard? Well, I may as well just have grabbed my laptop. At least with the laptop, I can multitask.
I know it sounds like i'm ripping on the iPad hardcore, but i'm just trying to do the OP a favor. These two devices overlap ALOT. Unless you have a very specific reason to get an iPad, its a luxury item. After the wow factor, you lose interest quick.
I do want to stress though, if you don't have an iPhone, then by all means its a great choice! If I were using an Android phone, it would so be worth it just to have access to the app store.