I often see business card printers asking for the customer to specify whether it's a portrait or landscape design. Is there any difference in the print process or notable difference by printing a portrait oriented design rotated 90º in a landscape format - if a printer doesn't give you the option to specify that it's a portrait design?
Having written some automation software for print shops in the past, I'd like to suggest another possibility. While both portrait and landscape may be printed exactly the same, your printer's pre-press software might not be able to detect the difference when generating the multiple-up sheet layouts they need for printing.
For example, a US standard size business card with a bleed of 0.125" on each side, would produce a file with dimensions of 3.75(H) x 2.25(V) in landscape, or 2.25(H) x 3.75(V) in portrait.
Putting a landscape file into a print template that has placeholders for portrait files, or vice versa, can sometimes cause problems.
Depending on what software they're using it can be pretty easy to write code that validates the dimensions of the file and rotates it as necessary, but it may simply be that they haven't done this yet and are required to put that data in manually.
Either way, I'd still suggest you ask your printer.
Alan Gilbertson is going to stop by any moment and join me in a rousing chorus of "Ask your printer." :)
As far as I know, standard business cards would physically be printed the same way no matter what the orientation of the type, but if your printer is requesting information, your best bet is to ask your printer what they need the information for and what difference it makes to them.