Lego has released software for controlling your NXT via a mobile phone. You can find the software and details here.
Documentation can be downloaded here.
Support is provided for the following:
Nokia: 6680, 3230
Sony Ericson: W800i, W550i, K610i, K800i, K750i, Z710i, Z550i, K510i
BenQ-Siemens: CX75, X75
Please let us know your results if you test the application.