At most scientific conferences and talks I have attended, the speakers generally present black text on white backgrounds, which I personally find rather dull. Is there any reason / explicit convention which should stop one from presenting light coloured text on a dark background?
Conferences tend to take place indoors in dimly-lit rooms. Dimly-lit rooms are a good place to fall asleep. Using a white background helps avoid this. Also, dark text/lines on a white background are easier to read (I've seen more than one study showing this).
If you are going to be showing a lot of astronomy or fluorescence biology images, where the image is mostly black with some interesting colorful things in it, you probably want a black or dark background for at least those slides; at some point you should just switch over. Also, if it is essential to understanding for the viewer to discriminate many shades of color, it's easier with a black background because you can use a wide variety of discriminable pastel colors that would be washed out to invisibility on a white background. You can't even use saturated yellow or cyan on a white background and expect it to be seen.
But for most scientific presentations there are good reasons for a light colored background.
In HCI/information science many conference presentations, especially in the best conferences like CHI or CSCW, tend to have nice colored backgrounds. I would even argue that in certain sub-fields of HCI or information science, just having a vanilla black or white colored background tends to be the exception rather than the norm.
This is a brilliant compilation of best paper awards in many sub-disciplines of computer science. A simple google search will often yield the conference presentations and slides of them. Under CHI, you will find that very often, the slides are rather innovative when it comes to the color scheme.
Most private companies have their specific color schemes and logos that must be shown on every slide. That's just the way it goes in industry. So it's just a matter of hitting the right conference :). I am pretty sure that at communication science conventions, you won't see any single white background presentation (or black background, for that matter).
Ideally, the color scheme should be a nice touch to your presentation, not a decisive point. The content should play the more important role, and the font size is arguably more important than the color it shows in: if nobody can read your font 6, what's the point of the slide? On some ways to make your presentations more effective without using multiple nested hierarchies of bullet points, see this multiple award winning one.
Whether light foreground on dark background or vice versa is easier to see depends on the light conditions of the presentation room.
Projectors are still not very powerful: the white of a projector will usually not stand out even against a comparably dim room illumination.
If it is really dark, bright foreground on dark background allows to show more shades of colour and brightness. Because the projectors are not too powerful, the risk of uncomfortably bright foreground is not very high.
If the room is not really dark, bright foreground on dark background may be very hard to see, as the eyes adapt to the overall light conditions and few projectors are powerful enough to make white text on a black (= grey because of surrounding illumination) stand out enough to be easily readable.
Note how the projectors becoming more powerful allowed a transition from white-on-black in really dark rooms (which were needed because the white was not that bright) to black-on-white in rooms with a dimmed overall illumination.