Donovan says: "gotta learn more about gettin' my xoxo on. i've read your page, les but still i have no idea what xoxo is or why i'd want to convert my opml files to it."
Good question, with a few different answers. And these are, of course, all my humble opinon and not any sort of official decree from the Gods of XOXO.
First answer I have is this: Don't convert your OPML files, because ideally a pair of XOXO-colored glasses should help get your OPML seen as XOXO, should the need ever arise. The reason you shouldn't convert is because you've got an OPML editor that's helping you be productive, and as far as I know there's no comparable tool for XOXO. Here, I think the utility of the tool trumps the format - especially when the format can be massaged by machines down the line, and not by your valuable brain. Also, someday soon, I predict that someone who likes XOXO will write a tool for the OPML editor that can export XOXO.
Second answer is this: XOXO is a format based on XHTML. XHTML is a format with which lots of other people have been productive, and it's directly viewable and CSS-styleable in web browsers without much fuss. This is the idea behind microformats: a Sudoku puzzle solution to the problem of publishing data that's both human and machine readable on the web at large. This is coming at the problem from the ground up - formats and data first, tools later, for the sake of imbuing the format with as much clear expressiveness as possible to enable future pathways.
I think right now, XOXO is more useful for people consuming the data than for people producing it. And, OPML is easier to produce than consume in some cases (but not all).
Where I think XOXO will really shine is in embedding more complex items of microformat data, something that I don't think will ever be a core feature of OPML. And that's okay - OPML will be good enough for capturing a lot of human brain output and it can be munged into XOXO whenever we need it as such. So, I'm happy to rest in the center and not become a cheerleader for either/or.