PhoneGap plays by the rules. If a vendor releases an SDK for a single operating system only, then you will have to use that OS to build and deploy your applications.
In detail, for each PhoneGap platform:
You can develop Android apps on any of the major desktop operating systems—Windows, Mac OS X, or Linux
You can develop Symbian Web Runtime apps on any OS but you can only run the simulator from Windows
You can develop apps for BlackBerry on any of the major desktop operating systems—the SDK can be installed on Windows or Mac OS X (to run the emulator, you need to install the virtual machine distributed with the SDK)
The iOS SDK requires OS X 10.7 or later (and, according to the OS X EULA, a Mac computer as well)
Note
You can emulate apps in the desktop browser with Ripple (a Chrome extension that is currently incubated in the Apache Software Foundation http://incubator.apache.org/projects/ripple.html) or with the online emulation service available at http://emulate.phonegap.com.
Practically speaking, your best bet for mobile development is to get a Mac and install Windows on a separate partition that you can boot into, or run it in a virtual environment using Parallels or VMWare Fusion. According to Apple's legal terms, you cannot run Mac OS X on non-Apple hardware; if you stick with a Windows PC, you will be able to build for every platform except iOS. However, you can still use OS X by using VirtualBox and running OS X from a Windows PC.
Anyway, with the new CLI utilities, it is getting to be pretty easy to build an app for all major mobile platforms. Mobile developers are well aware of the problems involved in building cross-platform apps; not surprisingly, the http://build.phonegap.com service is starting to become pretty popular due to the fact that it lets the developer use his/her favorite operating system. After registering with this service, it's possible to build a cross-platform app starting from a common code base. You can upload the code base or pull it from a GitHub repository. At the end of this book, I have included a section dealing with the distribution process for mobile applications.