You are generally considered married for tax purposes as long as you were married as of the last day of the year, regardless of whether your marriage license has been issued. Your last name is a non-issue, as many married couples file jointly even when they have different last names.
The decision about whether to file joint or separate returns depends on which would be better for you from a financial perspective. The best way to make this determination is to have your tax preparer prepare a draft of both types of returns (single and married-filing-jointly) so that you can determine which produces the better results for you and your spouse.
For more insight, see Happily Married? File Separately! and The Tax Benefits Of Having A Spouse.
This question was answered by Denise Appleby.