Is there a good hotkey cheatsheet for using Latex in Emacs? A few minutes of googling didn't turn up anything.
I don't think the TeX mode shipped with Emacs has a cheatsheet (or a 'Refcard' as they're often called in the Emacs world). The closest thing around for AucTeX is its fine manual (PDF here, the quickstart stuff begins on about p. 18). Also note, RefTeX, a full featured citation manager for your BibTeX database (docs here). My own favorite citation manager is the slimmer Ebib.
Another great Emacs-Latex tool is org-mode. It works as a mode for the efficient outlining, writing, and editing of documents in its own lightweight markup syntax. This can then be converted to a few different formats, including LaTeX. Many of us do our writing in org-mode (which does have a cheat sheet, here) and then convert it to LaTeX. Hard stuff may have to touched up by hand in LaTeX, but org gets the basics done nicely, and the 'basics' even do a pretty good job at things like tables, math, poetry, document structure, etc.
When in doubt: C-h m for help on the current major mode.
This one might be helpful. I used the US Letter PDF.