Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_2_1 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8C148a Safari/6533.18.5)
First and foremost Steve said he's requesting a leave of absence which indicates that he believes he'll be returning at some point in the future to resume his day to day duties. His remark about The plans they have for 2011 certainly would indicate that he expects to be gone for much of the year.
IMO there is no reason to panic. Tim Cook proved during Steve's last leave of absence that he can handle the day to day operations perfectly well and Steve has said he'll be involved in any major strategic decisions so I don't see how Apple misses a beat this year. Any major product releases scheduled to occur this year (iPad 2, iPhone 5, OS X 10.7, Mac product refreshes, etc) are already in the pipeline and have been for some time.
Worst case scenario, if this is permanent, as long as Steve's around he'll be involved in major strategic decisions as he said he would be and there's going to come a time at some point in the future where he's not involved with the company at all. What's important is that he's developed a culture of innovation at the company and that it continues. It's not like Steve's being forced out and will have no say in Apple's future. Tim Cook is his hand picked successor.