Ford announced this week that production of the 2009 Mustang will end early - less than a half model year. That means the restyled 2010 Mustang will arrive well before the year 2010 - probably around March of 2009 so that it will compete on day 1 of the Camaro's availability. But will it be competitive?
The 2009 Mustang glass roof option is shown below. The glass roof can be offered on the 2009 model because the roofline of the 2010 is identical - so no changes are necessary. The rest of the 2010 is simply a "top hat" restyle, as it's known in the industry. The measurements of the car are identical, all hard points are identical. External sheetmetal is updated and there are slight updates to the dashboard and interior: ( http://www.drivingenthusiast.net/sec-blog/2008/06/04.html#a2414 ). These kind of skimpy updates are all that can be done when when your company is unprofitable, your corporate HQ is in hock to your banks, sales are way down (especially of the "sugar daddy" F series), and development money is extremely tight.

2010 V-8 Mustang
It remains to be seen what happens to the powertrains: alledged or self-proclaimed "insiders" repeatedly contradict themselves as to the 5, 5.8, and 6.2 liter engines and their availability.
Here's hoping that the 5.8 and 6.2 liter engines are abandoned... and that the 5 liter 4-cam receives variable valve timing and direct injection. Yes, I know, where the heck is development money suppossed to come from for that kind of update? Where else would that engine have to appear?
2010 V-6 Mustang
The largest proportion of Mustang sales are V-6 models, and it remains to be seen what will happen for the 2010 model. Rumors suggest that plans to switch to the modern 3.5 liter engine have been moved out, and that the 4 liter will receive slight updates and soldier on for a few more years. Par for the course, since that engine first came out in the early sixties and only received significant upgrades in 1999 when it moved to new SOHC heads and structural updates. Look to the 2009 or 2010 Explorer for an indication since the Mustang is forced to share the same engine for cost reasons and the Explorer itself soldiers on for a few more years in it's present form).
So, it will probably be ye olde 4 liter V-6 - which is not at all competitive with the direct injected 3.6 DOHC V-6 Camaro engine http://www.drivingenthusiast.net/sec-blog/2008/05/22.html#a2403 and never can be. Note that the 2009 Challenger is in little better shape... it's extremely out-dated SOHC 3.5 liter V-6 only makes 250 horses. It's due to be replaced within 2 years by an all-new state-of-the-art DOHC V-6 engine. Dodge is also counting on at least 2/3rds of all Chalelnger sales to be V-6 powered.
Will Ford spend the money to build a competitive V-6 Mustang???
If it is the 3.5 DOHC V-6, then the Mustang is in the ballpark of competitive. The 3.5 liter V-6 was also developed in rear-wheel drive format right from the start. In fact, my website showed the first picture of that engine - almost 3 years ago!

See my website's section on the new 3.5 Duratec here: http://www.drivingenthusiast.net/sec-ford/FMC-engines/6m35d/ (suggest <right click> and <open in new tab>).
And then there is the upcoming "EcoBoost" twin-turbo version of this engine: http://www.drivingenthusiast.net/sec-ford/FMC-engines/6m35d-ecoboost/ - which is why I believe that while there might be a 5-liter or an updated 4.6, there won't be anything larger. The ~350 HP EcoBoost arrives in calendar year 2010 and has already shown itself to be capable of over 400 HP.