Can my fiancee visit the US while I-129F is processing?

I'm on the same boat as you with the I-129F. I filed in September 2007 and am still waiting for the NOA 2. Anyways, if she has another valid visa I believe she can visit. But the best site in the world for discussing and learning about the visa process is http://visajourney.com/

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She is welcome to apply for a B1/B2 non-immigrant visa, but the decision to issue is entirely up to the Consular officer, and the decision to allow entry into the US is entirely up to the CBP officer. Since the petition has already been filed, she is a defacto intending immigrant, and therefore would generally be considered ineligible for a non-immigrant visa. However, an immigrant petition and a non-immigrant visa are not always mutually exclusive. Unfortunately, and you may find this hard to believe, some people, thinking themselves exceedingly clever, actually try to use a non-immigrant visa as a way to get the applicant to the States and hang out until the immigrant visa is ready, thereby effectively bypassing the waiting/processing period for the immigrant visa. Many people with legitimate travel needs are denied visas out of hand as a result of such manipulations by others.

I'm not sure she can, because this would show immigrant intent. The only way that people are lawfully admitted to the US as visitors is if they do not have immigrant intent. I would call or email the embassy in London and ask them.

Alternatively, the two of you could meet up in Canada.

Yes, she can visit USA