reading an interview with her or something when The Time of Angels/Flesh and Stone came out, and she did say something to the effect of not wanting to flirt as much with Matt Smith because he was so much younger than her and she didn't want River to come off as a cougar. I don't think his Doctor seems that young when he's in character anyway, but it's understandable.
Makes sense, I suppose. But you're right that Matt Smith does a good job of seeming a lot older than he is.
In a more meta/continuity approach, the River we've been seeing this season is apparently much younger than the River we met in the Library, so perhaps her different attitude can be attributed to that she's in a different place in her life. 
Perhaps... I can see that.
soilent brad is PEOPLE!
1 + 2 + 3 + 4 + 5 = EM2C <-- Define it!
"Uh...we don't support the Hannah Montana empire."
- My 6 year old niece
1 + 2 + 3 + 4 + 5 = EM2C <-- Define it!
"Uh...we don't support the Hannah Montana empire."
- My 6 year old niece
/Doctor Who: new trailer
- 20/01/2011 10:24:10 PM
789 Views
The Doctor in America makes me feel patriotic. 
- 20/01/2011 10:33:21 PM
591 Views
- 20/01/2011 10:33:21 PM
591 Views
Re: The Doctor in America makes me feel patriotic.
- 20/01/2011 10:44:04 PM
612 Views
- 20/01/2011 10:44:04 PM
612 Views
Have we a start date yet? *NM*
- 21/01/2011 04:05:59 AM
326 Views
No, but it is expected to be earlier than the last couple of years
- 21/01/2011 05:13:07 PM
584 Views
Saw this after watching the Christmas Special.
- 21/01/2011 06:42:20 PM
671 Views
Re: Saw this after watching the Christmas Special.
- 21/01/2011 06:43:51 PM
636 Views
Awesome 
- 22/01/2011 08:08:17 PM
508 Views
- 22/01/2011 08:08:17 PM
508 Views
Re: Awesome
- 22/01/2011 08:09:38 PM
554 Views
- 22/01/2011 08:09:38 PM
554 Views
Thing is...
- 22/01/2011 08:21:29 PM
596 Views
Re: Thing is...
- 22/01/2011 08:22:38 PM
667 Views
Re: Thing is...
- 22/01/2011 08:44:16 PM
653 Views