The .MOBI gTLD (Generic Top Level Domain) is a bit of a marketing ploy. The idea behind it is similar to other suggested gTLDs, like .kids or .xxx, whereby visitors who go to sites with those suffixes can expect certain things. In the case of .kids, they might expect content that was safe for children, and in the case of .xxx ... well ... something a little different.
In the case of .MOBI, visitors should be able to expect to access sites that are coded according to the developing 'standards' for mobile device access as defined by dotMobi (the 'registrar' for that gTLD). In other words, when you go to a site in the .MOBI TLD you can expect it to work with your cellphone. Same with mail servers under the .MOBI domain, according to their documentation.
The same trademark policies are in effect for this gTLD, i.e. you won't be allowed to purchase ibm.mobi unless you're actually IBM.
Personally, I find it a bit amusing that the domain for mobile devices adds a character to the length of the nomenclature. It's already a pain to type anything into a mobile device, and this adds one more stab at the keyboard.
Hope that helps!