If a recipe called for spring onions, I wouldn't substitute shallots or vice versa. I'm not a food nazi - just that shallots are far milder and I've only ever had them cooked. Spring onions are quite feisty and often eaten raw, or flash fried to retain their crunch.
Just to clarify - a spring onion or scallion has a tiny bulb, barely wider than the shoot. It had an intense almost spicy flavour, but is still acceptable enough to the palate to dip and eat on its own.
Ooh, found pics so we can be sure we mean the same thing
BTW there is a type of onion which looks like a spring onion on steroids - can't for the life of me remember what it's called. It's imported and personally I steer clear.