If I recall, a qualified dividend is taxed as a capital gain - 15% for long term (> 12 months), 35% for short term based on the length of time you held the security/shares in startup/corp/whatever. A non-qualified dividend is taxed at your base tax rate. Bear in mind that a dividend can alter your base tax rate, which affects contributions you may have made throughout the year. So, if you were in the 15% bracket and because of this payment get pushed into the 33% bracket, not only do you need to compensate for the cash received, but, the potential tax liability from your new annual base.

If you are given shares in a company after the initial date, I don't know if that resets the calendar, prorates it, etc.

In my case, as the sole shareholder of a closely held Maryland S-Corp, the IRS deemed the cash payment to be a qualified dividend from a company that was 7 months old, therefore, 35%. Overall, it took roughly 2.5 years and about $7k in accountant fees to reconstruct my bad bookkeeping and deal with the IRS, $1.5k in interest and penalties for an unpaid $2100 tax liability.

Again, if your income ever goes crazy for whatever reason, talk with an accountant.