Andy, the day when you can carry a world class language dictionary in your pocket has already arrived. I own an Ipod Touch. There are two different companies that offer as an application the most extensive English, Spanish dictionary available, the Oxford 4th edition English Spanish dictionary. This is what I use for my own study. It has many times more definitions than any pocket dictionary, complete with many full sentence samples for virtually every word.
In paper form, this dictionary probably weighs several kilograms. But it is all available at the touch of a button on an Ipod. The interface is great, also, way better than a paper book. You have back and forward buttons, bookmarks, can click on any word in the definition and instantly that words definition pops up, etc. It is much faster to look up a word from scratch electronically than in a big paper book. The cost is $25. There are also a bunch of cheaper language dictionaries available. And even some free ones. Since I am a serious student, I opted for the heavyweight dictionary. This has also made it easier for me to do my Spanish studies at the mall, in the hotel lobby, anywhere. Reference material is one area where an electronic interface is clearly superior to a paper book interface, independent of the portability issue.