In a note to clients issued Monday morning, Canaccord Genuity’s T. Michael Walkley offers a snapshot of what he thinks the tablet market will look like by the end of next year. Overall, he expects unit sales to more than double, from 20 million in 2010 to 55 million in 2011, as a host of rivals scramble to compete with Apple’s (AAPL) iPad.

But none, in his opinion, is likely to catch up.

“Given the iPad’s time-to-market advantage, superior user interface, and leading application ecosystem,” he writes, “we believe Apple will maintain both market share and value share leadership in 2010 and 2011 for this new product category.

“Based on leading content for its iPad device, we believe demand for iPads will likely remain ahead of Apple’s ability to supply iPads in [the second half of 2010]. Should iPad production continue to ramp, we believe our 13.4M estimate for 2010 and 25.5M estimate for 2011 could prove conservative.”

Handicapping the competition, Walkley anticipated the launch of Research in Motion’s (RIMM) 7-inch tablet [UPDATE: dubbed the BlackBerry PlayBook] at DEVCON 2010 in San Francisco Monday and Nokia’s (NOK) Symbian^3-based MeeGo later this year.

But Walkley expects most of the competition will gravitate toward Google’s (GOOG) Android, which he sees as problematic.

“While we anticipate several Android-based device launches in Q4/10 and 2011,” he writes, “we believe these tablets could struggle to sell versus the iPad. We believe there is some level of confusion within the Android ecosystem of the ability of Android 2.2 (codenamed Froyo) to run successful tablet products. In fact, Hugo Barra, director of products for mobile at Google, recently indicated Froyo was not optimized for use on tablet-like devices. However, neither he nor any other Android representative from Google provided clarity as to whether Android 3.0 (codenamed Gingerbread) will be designed specifically to handle tablet-like devices.

“We believe this is consistent with our thesis that the Android ecosystem is gaining traction in the smartphone market, but we remain concerned about the stickiness of the overall ecosystem and the inability of Android-based OEMs to control product launches given the uncertainty around Android OS iterations. The tablet market is a case in point, where several Android-based handset OEMs were ready with tablet devices but still need to wait for Google to provide the necessary updates needed to optimally run Android on these tablet products.”

Below: Walkley’s list of the tablet computers he expects to be battling it out this year and next.

[Follow Philip Elmer-DeWitt on Twitter @philiped]