We chose the K1 option, my fiance is Canadian and I am the USC. Before deciding to proceed by the way of K1 we also considered the option for my fiance to get a job here and come on a work visa so we could be together right away. The K1 seemed more honest and straight forward, though longer of course.

To get to the point, right now just out of curiosity: let's suppose a Canadian citizen is in the US on a work visa, what happens if you fall in love, get married to a USC???? do you have to go back to Canada and apply as a spouse or can you stay in the US and adjust your status here?

The reason we did not do it was that we were afraid it would be considered fraudulous if they caught on and probably totally ruined our chances as we would have been living together from the very beginning.

Thank you guys and girls,

Partly dependent on which type of work visa.

For H1-b, either AOS (you can file I-485 ONLY, as H1-b can be extended if near-expiry in 1-year increments; if not near-expiry, it can be used AS both work-permit and travel-reentry permit). Also, marriage can be "safely" (more explanation below) be done outside of US.

For TN-1, it is best to have the marriage IN US and do AOS (full-package--and NO leaving US until AP in hand). In theory the marriage could be done outside but there is high risk of reentry denial by inspecting official (marriage to USC is "red-flag" of immigrant intent). Or, consular processing (CR-1), so long as holder is absolutely sure of not travelling outside US till just before interview at Montreal (for the converse, travel outside US while CR-1 is in process could result in reentry denial at POE/PFI).

Can't answer the cases for L-1 or R-1.

(I have been on TN-1--though never married in that status)

There is one problem... a TN is not available to those who have immigrant intent (which you've already tipped them off that you have by filing the I-129F petition) If you were to make a misrepresentation to the CBP in the coruse of getting the TN status, it could come back to haunt you. (Witholding information when a question is asked is also considered a material misrepresentation)

However, the scanario that you describe... if a Canadian got a TN for work.. met someone while they were in the US, got married.. then yes, it would be appropriate for them to file the I-130 and I-485 for AOS in the US. In this case, there was no immigrant intent on the part of the alien when the status was given... the immigrant intent happened only after the alien was already in the US...