I used bitinstant a couple times back in March / April and they were pulling the same stunts back then - accepting cash and not delivering the BTC for days or weeks later, all while feeding their customers BS excuses. They really should have a banner on their website that warns new customers about the delays, but they won't put one up because they're shady like that.
Ultimately you will get your BTC, but you just have to keep riding their asses. Keep emailing them and keep posting in the support thread here on bitcointalk a couple times per day.
Or create a post in the bitcoin section of reddit and hope it gets upvoted enough that they're publicly shamed into pushing your order through... I've seen this work at least a dozen times before.