Perhabs Tahoma is a TrueType font, and you need to register it first. According to the user guide of ReportLab you need to do this:
from reportlab.pdfbase import pdfmetrics
from reportlab.pdfbase.ttfonts import TTFont
pdfmetrics.registerFont(TTFont('Vera', 'Vera.ttf'))
pdfmetrics.registerFont(TTFont('VeraBd', 'VeraBd.ttf'))
pdfmetrics.registerFont(TTFont('VeraIt', 'VeraIt.ttf'))
pdfmetrics.registerFont(TTFont('VeraBI', 'VeraBI.ttf'))
canvas.setFont('Vera', 32)
canvas.drawString(10, 150, "Some text encoded in UTF-8")
canvas.drawString(10, 100, "In the Vera TT Font!")
The canvas object has a getAvailableFonts method that should return all currently registered (and therefore usable) fonts.