If running Windows on your Mac isn't cutting it for you and you need access through your iOS device, Parallels has released Parallels Mobile for the iPhone, iPod touch and (most appropriately) iPad.
From the looks of things, Parallels Mobile is a tightly integrated RDP or VNC client with the desktop software acting as the server. You use your Parallels account (which you register on the first launch of Parallels Desktop 6) to sign in on both your local machine and your iOS device so Parallels can circumvent all the port-forwarding you'd otherwise have to deal with. Once you've got all of your devices synced up, you'll see a list of your virtual machines. A tap of the play button on your iOS device will activate your chosen virtual machine on both the iOS device and your local machine.
Once the virtual machine is up and running, you can zoom in and out with the usual pinching multitouch gesture, click things with a tap and swipe around the screen to get to where you need to be. Parallels Mobile provides a means of watching Flash videos on your iOS device, but I wouldn't call it a great way of going about it. Regardless, it's pretty neat to be able to run Windows on your iOS device with such little effort.
Parallels mobile is a free app for all iOS devices, but it requires a copy of Parallels, which will set you back $109 new or $69 for the upgrade.
Parallels Mobile [iTunes App Store]