I don't know if anyone else has noticed this but recently we discovered quite a spike in yahoo.com addresses that came back as hard bounces. As we looked a little closer we found that these hard bounces all occurred on August 1 - 10. Some people were double opt ins, others were not. Not all yahoo.com addresses came back as a hard bounce - which has me scratching my head.
When I initially contacted support they said it was a known issue - that yahoo.com had hard bounced a number of email addresses during the first week of August. They said they would search for such cases and put them back into the system (remove the hard bounce tag) which they did. I was told to let them know of further issues. Well we found about 1800 such issues and this time tech support isn't so sure what the problem is but I'm hoping somebody will look into it and correct it.
You might want to check your yahoo.com hard bounces and see how many are actually hard bouncing and did so during the first week of August. You might be surprised at what you find and hopefully Infusionsoft can correct this issue. I figure the more people that point it out the higher the likelihood that it will get resolved and possibly sooner.