You are perfectly free to license the software under GPL, and under a commercial license. The GPL's copyleft requirements do not apply to the copyright owner, only to people who need the GPL to be allowed to copy and modify the software.
That's how dual licensing works.
Of course, for details you'd need to ask a lawyer, but the Wikipedia page is a decent introduction: http://en.wikipedia.org/wiki/Dual_licensing
To clarify my statement that "the GPL's copyleft requirements do not apply to the copyright owner": This is just basic copyright law. Under copyright law (and related law), the copyright owner has all rights. Releasing code under any license (such as the GPL) just grants some of these rights to others, along with any restrictions the copyright holder wants to attach.
The restrictions imposed by the GPL are only binding because without a license you would not be allowed to copy and modify the software at all - because of copyright law. This is the idea behind "copyleft". All this, of course, does not apply to the copyright holder.
Straight from the GPL FAQ from the FSF:
Q: I would like to release a program I wrote under the GNU GPL,
but I would like to use the same code in non-free programs.
A: To release a non-free program is always ethically tainted, but
legally there is no obstacle to your doing this. If you are the
copyright holder for the code, you can release it under various
different non-exclusive licenses at various times.
One more clarification: The whole dual-licensing thingie only works as long as you are the copyright owner for all the code. As soon as you start accepting code from others, you will be bound by whatever license they choose for their contribution. I.e., if you accept GPL-licensed code into your codebase, any version that includes these contributions must be GPL-licensed as well.
The only way around this is for contributors to assign copyright to you - that's why many organizations require copyright assignment from contributors (e.g the FSF does this). Of course contributors might not want to assign copyright to you - in that case you'll have to convince them (e.g. by paying them), or leave their contributions out of you non-GPL version.