According to the following press release from November 2008, Flash 10 on ARM (ARMv6 and ARMv7architectures used in the ARM11 family and the Cortex-A series of processors) is expected to be available in the second half of 2009.

http://www.adobe.com/aboutadobe/pres...dobeFlash.html

Chances are it was too late for integration and testing with Maemo 5/Fremantle, or it wasn't sufficiently stable to be considered for Maemo 5 and could have significantly marred the Maemo 5 experience, so was dropped in favour of Flash 9.

Finally, according to the following press release from April 2009, a pre-release of Flash 10 should have become available in the summer optimised for OMAP, with a view to production release in Q1 2010 and rolled out to devices in H2 2010.

http://www.adobe.com/aboutadobe/pres...obeTIOMAP.html

Obviously Nokia wouldn't want to risk a pre-release Flash 10 on Maemo 5/Fremantle, or it arrived too late to integrate into Fremantle, or both.

I suspect there's a good chance Flash 10 will arrive as an SSU (non-optimized first, followed by OMAP optimized) sometime late in 2009, with more updates later in 2010. The OMAP optimized Flash 10 should be available for Harmattan.