Self-plagiarism and supplementary information
Methods sections (supplementary or otherwise) are one of the few places where self-plagiarism may not apply.  This is because the typical format for a methods section is not narrative (for which creativity is expected) but a highly compressed sort of recipe (e.g. "DHT5a cells in LB, grown in 96-well plates, 150 mL per well, at 37C for 18 hours while shaking at 300 rpm.").  In this form, method sections are essentially a computer program to be executed by a human, and as such the question of self-plagiarism is irrelevant, just like it is if you attach a computer program to your supplementary information.

If you have a "long-form" method section, however, where you give a narrative explanation of an approach, its reasoning and justification, how it relates to the experiments, etc., then the narrative aspects are expected to show originality and self-plagiarism does apply.  This is because you have explanatory text that should be contextually adapted to the rest of the paper, an using it in a new paper changes the context and should thus change the text.

For an example illustrating this distinction, here is the supplementary section from on of my own recent papers.  Section 9, "Additional Methods" is a "recipe section" for which I would not be concerned with the question of self-plagiarism.  All of the other sections are "narrative sections" where one does need to be concerned about self-plagiarism.