If you tried the newest version of SMF (2.1 beta) and you want to move to phpBB 3.1, you have to do some tricks in order to convert successfully your board.
First you need to download phpBB 3.0.14 from here and install it. Then download the SMF to phpBB convertor for SMF2.0RC2 from here.
Extract the contents from the convertor and open the file "convert_smf20.php" with a text editor. At lines "406" and "418", change this:
array('rank_image', 'membergroups.stars', 'smf_rank_image'),
to this:
array('rank_image', 'membergroups.icons', 'smf_rank_image'),
Last, in the same file, comment out the lines "751, 765, 767, 768, 769, 770, 771, 786". Now, upload the files to your server and start the conversion.
Next, you have to upgrade from 3.0.14 to 3.1. Follow the instructions from here.