Warning to readers: U.S.-centric content follows.
While a number of English-only movements are clearly rooted in hostility towards America's largest immigrant population, there is another way of thinking about it that (IMHO) has some legitimacy: Without all of us being able to operate in a common language, civil order and equality of economic opportunity cannot help but suffer.
There is genuine benefit in standardizing on a single language for all Americans to conduct their civil business in, whether that is voting, promulgating a lawsuit, or what have you. Without that, we wind up being two (or more) countries in one that cannot hold a meaningful dialogue with one another. Inevitably, one group will be more powerful (currently those that speak the dominant American English dialect), and the other will suffer from poorer access to civil services and economic opportunity.
What such a single-language system needs, and what many English-only proposals lack, however, is sufficient educational opportunity for those who speak the non-dominant language (or non-dominant dialects of English, such as those spoken in Appalachia or predominantly African-American parts of major cities) to learn the dominant English dialect. This may be the hallmark of an un-bigoted English-only program - the willingness to spend the money and effort to bring the linguistically disenfranchised into the fold.
Some will whine that this is a way of assimilating and destroying elements of foreign cultures that are brought here by immigrants. I don't buy it: First, nothing keeps people from using their own language and expressing their cultural uniqueness in homes, churches, and neighborhoods. Secondly, people who move from one country to another are going to have to expect to make some concessions to their new environment. I wouldn't dream of moving to France or Japan or anywhere else without attempting to learn enough of the language and culture to get by.
Anyway, that's my US$2e-02.
http://www.bradheintz.com/ - updated kind of daily