I have bought pink sheet share of a company called bankunited. The company announced that they will be going IPO. How can a company have shares for sale (penny stocks via pink sheets) then go Ipo? What does this mean for me?

It means the value of your stock is about to go down. I would guess that the company is floating a new stock issue in order to raise cash, so the percent of the company that your stock shares represents will be diluted, leaving you with a smaller ownership interest than you had before. Consequently the value of your investment is reduced.

By the way, according to their web site Bank United is privately held, so I don't understand how you have shares in the company. Is this it?: https://www.bankunited.com/about-us.aspx

That is the correct bank. They released a portion of the stocks for sale. You can buy it as a penny stock right now.