IE doesn't seem to be able to change text size when the text is specified in pixel sizes, something they STILL haven't fixed in v7. I usually make sure text is large enough that most users would never need to resize it, and I would consider your text as too small - I don't use anything under 12px.
actually that is correct text size shouldn't change when its set to PX as PX is a static width the size of a pixel doesn't change so logically this is correct of IE but in practice this is wrong.
you should define a font-size in the bode tag in PX but then all your text should be contained within the correct tags <p>, <h1> etc... which should have font-size:**em; which will make your text size dynamic.
i have just started to use EM's for all fixed width layouts as it allows the site to expand with the user if they need a bigger font size