http://en.wiktionary.org/wiki/Taser says tasered.
Someone else says:
Taser is actually a trademarked name, and the company, Taser International, also prefers tase as the verb. The company website contains multiple instances of phrases like “the subject was tased and incapacitated.” I also called the company and a friendly customer service rep named John confirmed that when they talk about incidents internally, they say someone was “tased.”
On the other hand, a Google News search produced about 1,800 hits for tasered and only about 200 hits for tased. So popular opinion is definitely on the side of tasered.
(I just did a search on yahoo.com and found that those numbers are now 1,090,000 for tased, and 3,960,000 for tasered. On google.com, the numbers are 251,000 and 792,000, respectively. Tasered is still more popular.)
The person continues:
I'm surprised by the popularity of tasered. If you use LASER as a model for TASER, you should come up with tase as the verb because lase is the verb form of laser. Both words are acronyms, so using laser as the model seems like the obvious choice.
(The web has more hits for lasered than it does for lased.)
One of the last comments by that person:
The “best headline” award goes to CBS affiliate WSLS in Roanoke, Virginia, for “Tased and Confused,” but even they seem confused because even though they use tased in the headline, they use tasered in the article.