Is a scheme always negative in the US?
In the US we do hear of other meanings. "The scheme of things." "A rhyme scheme." However, the verb "scheme" (according to the OED) is mainly negative nowadays, even in England.
I agree it often has a negative connotation.
If you say that someone is "scheming", as in, "Jack is scheming to ...", that's pretty much always negative.
But, "I have a scheme to fix our production problem", or "What's the scheme for marketing this weak?" don't imply anything underhanded. Of course if you describe a scheme in negative terms, it's going to be negative, like, "Bob devised a scheme to ruin Sally's reputation."
I guess like many words it depends on context. But given that it is used in negative terms more often than in positive terms, I'd generally use an alternative word for something positive, like "plan".
I would say it is usually negative, but not always. In my experience, if someone in the US says they are scheming to do something, it is tongue-in-cheek.
Lately the word is often used in the US after Ponzi in light of the Bernie Madoff investment scandal.
Scheme is used often in technical papers, to describe algorithmic approaches to problem solving. In that context, it has no negative connotations whatsoever, and is as benign as the previously-cited example, rhyming scheme. Some examples:
"The selection of a particular numbering scheme is based on the analysis of the corresponding adjacency matrix." (Kier and Hall, Molecular Connectivity in Chemistry and Drug Research)
"The simplest scheme to solve this problem is to make separate copies of the goal's arguments for each alternative clause, but this is very inefficient. To reduce the copying overhead, variant schemes based on the idea of shared environments have been proposed." (Yang, P-Prolog, a Parallel Logic Programming Language)
"The selection of a numerical scheme to solve practical fluids engineering problems is still an art." (Valentine, Control-Volume Finite Difference Schemes to Solve Convection Diffusion Problems)