There is another advantage to waiting,
OS X Mountain Lion 10.8 is going to be released later this summer, so the next new iMac released after that would be the one to buy.
The advantage of this is, especially for a new to Mac owner, is that you get the benefit of a new OS, on new hardware combined with Apple holding your hand for three months for free.
If you buy now, you get the buggy 10.7 which hasn't been a very good release at all, then you'll need to upgrade OS X to 10.8 on older hardware, which also tends to lead to problems.
So why have problems when you can wait a few more months and have none?
There is some disadvantages to iMacs and Mac's
1: iMac's you can't replace anything inside except add more RAM, you cant' touch the hard drive with your data on it. Anything sensitive, even if you use Filevault, you need to use a external drive in case the machine breaks, you have to drag the whole thing in for repair.
2: AppleCare is a MUST on nearly all Mac's for three years of coverage because the repair bills will shock you to death.
3: Apple expects you to upgrade your operating system on their schedule, which is annually now, so it tends to break all your third party software and slows your machine down, if you don't upgrade, your denied security updates as Apple only supports the last two OS X versions in circulation.
On the contrary, despite more malware on Windows, Microsoft does support their OS versions for apx 10-12 years and you can replace the OS from System restore images and apply patches, install new software. Mac's you can restore, but not apply new patches or new software on older OS X versions.
4: You need to employ a multiple backup plan with Mac's, and try not to get "trapped" by your data so you can use it on PC's if you find out Mac's are not to your liking.
5: You must understand that with Mac's there is a LOT of change, you can walk into a AppleStore a year later just to find out you can't get or use a new monitor with a old system, have to buy a $30 adpater or some other thingy to get your old hardware to work with newer ones.
6: You will find out there isn't a whole lot of hardware choices, that Apple constrains you quite a bit and there isn't as much software selection on Mac's as on Windows, however it's usually of a better quality as much of the Mac hardware usually is. If you need specialized hardware like a Toughbook, then you need Windows, Apple has a very limited selection of hardware, take it or leave it.
7: Understand that your going to be entering the "luxury consumer computer market" basically the Gucci of computers and be paying a lot more for your computers, tech etc., than with a Windows machine and not really get any value, just a better experience. With Apple, if your machine or OS X is about two years old or older, then your frowned upon as not being hip enough to keep up.
8: If you need to work a LOT with Windows machines, it's likely better to get a Windows machine. If some, then you can use Windows in Bootcamp or a virtual machine software., Less then you can use Office products on a Mac.