Ok, not sure how header.php's usage has changed, but I found the problem lies there.
If I blank out that file (remove all my custom header stuff), the problem goes away, but the very same custom header that works under 6.5.5 on the very same server throws that error under 7.0.
**Edit: If I leave out everything from <? to ?> and leave in only the regular html, it loads just fine.... I'm obviously doing something wrong for how it's done now, just not sure what.