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[suarutsu]

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[Harisenbon]

But I've recently been studying a travel book, and there they say that it means 'can I help you?'.

Probably shouldn't use that book anymore.

For all accounts and purposes, it means "welcome (to the store/business/etc)."
There's really no reason to respond in any sort of way, and a response is not expected.

If you can't deal with not responding, a simple nod of the head or a slight こんにちは is more than sufficient.

[NileCat]

Harisenbon is 100% right though, there exists another magic word in Japanese that you can use as your reply to almost every greeting, anytime, anywhere.

「どうも」 (doumo/do-mo)

[AJBryant]

I always used to nod.

Man, I miss that cheery chorus of "irasshaimase!!!!" -- and my local shop guy's "Eh, rasshai!"

NATSUKASHIIIIIIiiiiiiiiiiiiiiiiッ！

[suarutsu]

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[keatonatron]

Thanks alot of helping out! =-) It's a good thing I asked then. I can't not-reply, so I guess I'll say konnichi wa, konban wa, doumo and such then

どうもありがとう
Rody

Or my favorite: うぃす！

[Harisenbon]

Or my favorite: うぃす！

あざ～すぅ

[Oracle]

A manager at the golf club I worked at once in Japan came into the restaurant part of the complex one day and was greeted by the girls serving there with the usual　いっらしゃいませ！ to which he replied はい、いっらしゃいました