Should form field values that are numbers be right or left aligned?

I just had an ex-colleague ask me this question and I couldn’t find any research on this matter. There is a lot on the alignment of labels in conjunction with form fields but nothing on the alignment of the value inside the field.

I chatted quickly with Luke Wroblewski about it (one perk of working in the same company) and we both agreed that if there are several fields, for example in a spreadsheet application, then monetary or generally number fields should be right aligned as that allows you to add them up quickly in your head without having to worry about decimals.

What are your thoughts, or do you know where I can find some research papers?