I'll assume you turned 70 on 8/25/09, and the 8/25/39 in your question is a typo. You will be 70 1/2 on 2/25/10. Therefore 2010 is the year of your first required distribution. Anything you might have distributed for 2009 is not counted as part of your 2010 requirement. For your first required year only, meaning 2010, you get until April 1 of the following year to make the required distribution. For 2011 and all following years, the required distribution needs to be made by December 31. If you do wait until 2011 to make the 2010 distribution, you will still owe the 2011 amount by 12/31/11.

Yes - that's true - I turned 70 on 8/25/08. Your answer is fine.
Now then - what percentage must I withdraw at 701/2?

Most people with the same set of facts as you will need to withdraw at least 3.77% of the 12/31/09 balance to satisfy the requirements. The RMD Uniform Lifetime Table found in the IRA Publication 590 indicates a factor of 26.5 for someone whose 71st birthday occurs during the given year. Dividing your balance by 26.5 gives the same result as multiplying by 3.77%. If your beneficiary is your spouse who is more than 10 years younger than you, you can use the joint and last survivor table to get a bit smaller required amount.

For this and most purposes, a SEP Ira is treated just like a traditional one. You calculate the required amount for each IRA and can distribute the total from any combination of IRAs you choose. That is for example, it could potentially all come out of only one of the IRAs. Note that the 401(k) is calculated just like an IRA, but each 401(k) plan must satisfy its own requirement.