There are a lot of rumours about based on the Sarah Jane Adventures press release and the Den Of Geek/DWM209 reports.
First the Den Of Geek report -
Russell T Davies says The wedding of Sarah Jane will be a day for everyone to remember!
And David Tennant will be starring in a two-part story, this is not a cameo appearance, he has a proper part in the story.
The press release adds to the puzzle -
...a chance for Sarah Jane to find personal happiness with someone who could be the perfect person to complete her family.
First up, the appearance of David Tennant may not be in the same episode as the personal happiness episode, although it is possible. Personally I'd have him in the season finale episodes.
Now lets remember Russell T Davies is a terrible liar, remember in series 4 he told us a companion was going to die, and Donna Noble just lost her memory instead?
Sarah Janes Wedding can mean many things, Sarah Jane Smith getting married, someone called Sarah Jane getting married, Sarah Jane Adventures itself marrying another show, such as Doctor Who for a story!
ATM its all very loose and under Russell T Davies control, could mean almost anything! After all the line beginning a chance for Sarah Jane to find... could mean Sarah Jane is buying a dog!