Sounds like you've already made your decision, but to reiterate lots of the opinions that have been voiced here already, trading in your laptop for an iPad is a really terrible idea. While the iPad can replace many of the functions of a laptop, it doesn't get anywhere close to the functionality. And just to give you a frame of reference, I own a windows 7 desktop tower, a 2010 13" macbook air, and an iPad 2.
-Web browsing: the iPad overall is terrible when compared to a computer. It's much MUCH slower at rendering sites. Tabbed browsing is basically non-existent in safari (though as others have mentioned 3rd party browsers can help shore up that deficiency). Lack of memory means that even WITH the tabbed browsing functionality of one of the 3rd party browsers, you're limited to only a handful (around 5 for me while using atomic browser).
-Flash! This is a huge one. If you need to access anything flash at all, the iPad becomes a paperweight. You'd be surprised how many sites out there use flash. Sure, most of that is silly flash games and website animations, but you'll eventually have some class that requires you to complete either homework or even an exam online, and for that you'll want a real computer and web browser.
-Youtube: strange that I'd even put this in here, cause for most situations the iPad actually works great for watching youtube videos. But there are still many videos that don't "work on mobile" - NBA.com videos just to raise one example.
-multitasking: try having a google talk conversation while...doing pretty much anything else on the iPad. HUGE chore. I use IMO.im (fantastic app) for my google talk needs on the iPad. If someone sends me a message, a huge popup interrupts whatever it is i'm currently doing. Tap close to quickly finish what i was doing. Now, I have to double tap the home button, swipe around the quick bar to find IMO.im, and tap just to start typing out my response. If I want to get back to what I was doing before, I then have to double tap the home button to bring up the quick switch bar. And most IM conversations are conversations - this entire process would normally have to repeat a hundred times, but usually at that point it becomes way too tedious that I would just switch to my real computer.
-the ecosystem: this is extremely important because part of owning a computer is how interoperable it is. Part of the reason why the iPad is so successful is its OWN ecosystem of apps and accessories. When you own a computer, you can now seamlessly work with anyone or anything that is also in that ecosystem. For example, someone gives you a file on a usb drive. On your macbook, you just plug it in to your usb drive and you're set. On the iPad, you have to first load the drive into your home computer, convert the file into one the ipad likes, and then transfer it over before you can watch.
TL;dr: If you want portability, sell the MBP and get a MBA. If you really want the iPad, keep the mbp, save up for a bit, and get like a refurbished iPad 1.