How can I set the WiX installer version to the current build version?

I did this in one of my projects by writing a preprocessor extension to read the file version from my executable. So the WiX file looks something like:

<?define ProductName="$(fileVersion.ProductName($(var.MyApp.TargetPath)))" ?>
<?define CompanyName="$(fileVersion.CompanyName($(var.MyApp.TargetPath)))" ?>
<?define ProductVersion="$(fileVersion.ProductVersion($(var.MyApp.TargetPath)))" ?>
<Product 
    Id="<product ID>" 
    Name="$(var.ProductName)" 
    Version="$(var.ProductVersion)" 
    Manufacturer="$(var.CompanyName)" 
    Language="1033" 
    UpgradeCode="<upgrade code>">

I've posted the code for in on CodePlex: http://wixfileversionext.codeplex.com/

You could use Product/@Version="!(bind.FileVersion.FileId)" (replace FileId with the Id of the file from which you'd like to get the version number) and light.exe will populate the value with the version of the file referenced by the FileId.