Hej
I have now also checked around a bit in abstracts to see if "we" is often used or not. My observation is similar to Filip's although not 100%. It seems like most use we and only once in a while people avoid using it.
So, you need not take this comment into consideration. My opinion is that abstracts should be a bit more formal and less personal than the main text, and as a consequence I try to avoid using "we" (but looking back I, or my coauthors, have used it once in a while ...). So do as you wish, but it is certainly not a problem at all to use it.
/Tom