You could insert a calculated page field in the header or footer. Press
Alt+F9 to turn on display of field codes. Put the the following field
in the header or footer:
{ = 0.5 * { PAGE } + 0.5 }
Don't type the curly braces; enter them by pressing Ctrl+F9. When
you're done creating the fields, press Alt+F9 again to switch back to
display of field results.
If you put this calculated page field in a regular header or footer,
then sequential pages will be numbered 1, 1.5, 2, 2.5, etc.
If you put this calculated page field in an odd-page header or footer,
then odd pages will be numbered 1, 2, 3, 4, etc., and even pages will
not be numbered. In Word 2003 or earlier, you can choose "Different odd
and even" headers and footers in File | Page Setup | Layout tab. In
Word 2007, look in Insert tab | Header and Footer group | Header &
Footer Tools | Design tab | Options group.