I have come to understand that "being in early music" really has many disparate meanings and various levels of meaning. The first and easiest level references the repertoire of music - generally speaking it is agreed that "early music" is the music before 1750.
The other meanings, though, become a little harder to pin down. For some, "being in early music" means that they perform on instruments that aren't quite so well-known today - the shawm, theorbo, or serpent, for example. For others, it means that there are slight adjustments to what we know as the modern version of their instrument - a violinist may use different strings or a different bow than a modern player, a flute may be wooden instead of metal, and an oboe may not have keys. For a singer, neither of these differences really matter - the voice is, more or less, the same as it always has been.
If the instrument may or may not be the same, this definition clearly isn't enough. So then, we dive into the most commonly imagined difference between "modern singers" and "early singers" - the discussion of technique. For whatever reason, there seems to be a belief that early music singing has its own technique separate from modern singing. The techniques required for singing early music may be different from what one would use to sing Puccini, but I would argue that the techniques used for singing Puccini, Ravel, Stephen Foster, or Charles Ives are vastly different from each other, even in the realm of "modern singing technique". Each of those composers is from a different country, a different time, and a different compositional style. Obviously, the performance of their works must be representative of the time and place it was composed and the performance practices of the time. One wouldn't dream of singing Puccini's "O mio babbino caro" and Ives' "Ann Street" with the exact same interpretation and techniques!
This is the same with early music. There is no one set of practices that can encompass the wide range of styles from the dawn of time to 1750, so even within early music there are many different stylistic choices that have to be made and many different techniques to use. Inevitably, when I have a discussion with another singer and they discover I am into early music, the question of "so you only sing with straight tone?" eventually comes up. For some reason, this seems to be the prevailing belief - that to sing early music, one can only use straight tone. Certainly there are times when a sound with less vibrato is desirable, but this is not to say that vibrato shouldn't be or isn't used. It is a stylistic choice - just like the use of dialect in Stephen Foster and George Gershwin or the portamenti in Puccini. This being the case, early music singers aren't really different from anyone else - we all make stylistic and technical choices based on the repertoire.
This isn't to say, however, that I could or should go sing Puccini or that someone who regularly sings Puccini can just pick up Machaut at the drop of a hat. They are drastically different styles, and out of respect, one should take the time to learn what is required of each before attacking it with no understanding of its idiosyncrasies of the expectations of style. The key point here is that there are many different techniques and each of us has better facility with some than with others. Within early music, some singers may excel at Renaissance French music, while others prefer Italian medieval and others German Baroque. Each of these requires very different techniques. The technique divide comes not between early music and everything else, but is much more subtle and divided across many genres, times, and countries. Even more importantly, good vocal technique is good vocal technique no matter what the technical demands of the repertoire.
Another difference commonly cited between early music and modern performers is that in early music, we take more time to understand the context in which a work was written and performed, as well as how other influences (art, architecture, the use/study of rhetoric, politics, etc.) may have influenced the composer, the performers, or the perception/reception of the work. Even if this is true, shouldn't modern performers be doing the same thing? Certainly modern orchestras wouldn't pick up Shostakovich's 7th Symphony ("Leningrad") without a discussion of the German siege of Leningrad during World War II (and whether or not the symphony even had anything to do with that). All musicians, early or not, should carefully consider the context of the pieces they are performing. While music has the ability to "stand alone", how much more significant is it when considered in the proper context? This should be an even easier task for musicians performing a more modern selection, since the historical record is bound to include more information on a more recent time. If this, then, is truly what "historically informed performance" means, shouldn't all performance be "historically informed"? If "early" musicians are those who research their music and the practices associated with it and seek to present it to an audience in an engaging way, shouldn't everyone be an "early" musician?