One can read about 3G (probably too much info) at [ame=http://en.wikipedia.org/wiki/3G]3G - Wikipedia, the free encyclopedia[/ame]. The elaborate on the iPhone part as the iPad portion has already been answered, the 1st gen iPhone did NOT support 3G. It only supported EDGE, at best. The iPhone 3G and beyond support 3G (see [ame=http://en.wikipedia.org/wiki/List_of_iPhone_OS_devices]List of iPhone OS devices - Wikipedia, the free encyclopedia[/ame]). Besides 3G networks providing more speed, the 3G networks on T-Mobile and AT&T allow for simultaneous voice and data. So, while on a 3G network, you can make and receive calls while actively transferring data. If one were to use 1st gen iPhone or turn off 3G on a iPhone 3G or 3GS (you'd be in EDGE or GPRS), if you're actively transfering data, you will NOT be able to make outgoing calls and your incoming calls will go to voice mail (phone won't ring). Also, if you have a voice call active, you won't be able to initiate any data connection (can't surf, download, etc.)
It has to be 250MB. 250GB is equivalent to a continuous 770 kbaud line, running full capacity 24/7. No cellular system can possibly afford to sell that sort of service for a mere two-digit price. [I come from the SI perspective, where the prefix 'm' means '10^(-3)', not '10^6'. Except for capacitors, where this prefix is forbidden until the generation of engineers who used 'mF' to mean microFarads has died off.]
An ibook probably would not use much bandwidth to download, but before you decide to get your textbooks on a computer, find a free ebook to download to your Mac and see if you really think you can read comfortably on a computer screen. While the e-ink technology of the Kindle and other dedicated e-book readers is like reading paper, reading on a computer screen can be very tiring on the eyes. And paging quickly through a textbook to find a reference may not be easy on a computer as it is with a paper book. I love my Kindle for reading novels, but I don't think I'd like it for reference books.
It seems to me Kindles et al would be ideal for reference books - just use the Search function and bang! You're there.
Paging around in a document is really rather awkward, and searching may yield too many hits. Also, the publisher may or may not included a good clickable table of contents, and page numbers are meaningless. It's great for reading novels or magazine articles. For anything else more navigation control might be needed.
