Armstrong also suggested he might be tempted to race in the 2010 season as well.
"I don't want to box myself in here," he said. "It's open-ended. I see one season but I wouldn't want to rule out a second season. I will take it season by season."
Armstrong, a survivor of testicular cancer, will start the six-day Tour Down Under race around South Australia, centring on Adelaide, on 20 January and is also planning a global summit to raise cancer awareness in Paris after next year's Tour.
"I look forward to 2009, I look forward to racing again," said Armstrong. "I cannot guarantee an eighth Tour victory, but I can guarantee you the 'Live Strong' message will touch all aspects of our society.
"It's not very often someone gets a chance to spend three or fours years away from something, step back, and then say to themselves, 'I sort of miss that, I'd like to go back and do that again."
With his career dogged by doping allegations, Armstrong will undergo a testing programme developed and headed up by U.S. anti-doping expert Don Catlin when he returns to cycling.
But the former head of the World Anti-Doping Agency Dick Pound demanded any testing should be conducted by an International Olympic Committee or WADA accredited laboratory.
"If it's not an (IOC) accredited laboratory the mere fact scientist X says 'I think Lance is ok' (means nothing)," said Pound.
The Kazakh-financed Astana team suffered two high-profile doping scandals in 2007 and were barred from this year's Tour de France despite a substantial overhaul in team management.
Kazakh rider Alexandre Vinokourov, the old Astana team leader, tested positive for blood doping after winning a time-trial stage of the 2007 Tour, and was subsequently sacked and banned for a year.
Armstrong return 'not great for cycling' - Boardman
Astana's current leader Alberto Contador, 2007 Tour de France winner and only the fifth rider in history to win all of cycling's three major Tours when he won the Tour of Spain, has previously hinted the American's presence could cause conflict.
But the Spaniard said suggestions he would leave Astana were "too premature".
"I am going to calmly talk to the team and depending on how it goes, we will see what we do."
And Armstrong said he was looking forward to racing with the Spaniard.
"Alberto is the best rider on the planet right now," he said. "We have to understand that, have to respect that. I'm not sure I can ride that fast right now. I hope it works out."
One rider who believes there could be problems for the Astana team is the Republic of Ireland's 1987 Tour winner Stephen Roche.
"I could see (Contador) walking away. He's finally getting credibility and now next year everybody will be talking about Armstrong. It's going to be very difficult," Roche told BBC Radio 5 Live.
The 49-year-old, whose racing career spanned 13 years, was also asked whether Armstrong had a chance of winning the Tour. "I don't think so," he replied.
Bruyneel said he was honoured and looking forward to working with Armstrong again, 10 years after being asked to direct the rider's US Postal Service team.
Bradley Wiggins on Armstrong's return
"What we saw from 1999 to 2005 was arguably the most exciting time in professional cycling and I know Lance will bring the same level of charisma, passion and influence to the team, sport and global cancer community," he said.
Armstrong nearly lost his life to cancer before battling back to win his first Tour in 1999.
International Cycling Union president Pat McQuaid told BBC Radio 5 Live he did not think Armstrong's "primary motivation" was to win the Tour.
McQuaid said: "I think he has done as much as he can with his cancer foundation in terms of knowledge of it in the US and now he wants to globalise the foundation using the sport of cycling.
"Winning the Tour is secondary but I think he has every intention of trying to do it."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.