I'm migrating two blogs from WordPress to Drupal 7. How would one handle migrating user subscriptions from the WordPress blogs? I've already used the Migrate module and it looks like that only handles content.
You may try using WordPress Migrate which supports migrating WordPress blog exports (WXR format) into Drupal using Migrate module. Check also Migrate Extras which provides additional support for contrib modules.
For any other extra functionality which is not covered, you've to extend it by writing your own custom class. Check Migrate Example for example of migration data.