I've found the easiest solution to be this:
You guys know those 3.5mm headphone jack splitters? (The things that let you plug two pairs into one jack)
They don't contain any diodes; they're just conductive contacts, meaning that current will go either way through them. Plug your speakers into the computer through one of those, then grab a 3.5mm extension cord (with the male bit at both sides; the bit that goes in) and plug it into the other, then run it up your desk and plug your iPod into it. It shares, so both components go to half loudness, but just turn it up a bit and you're good.
As in:
Computer audio jack <----speakers
----iPod
Hope that made sense :lol: