How to escape commands in a bashrc alias?

The shell expands the command line containing the alias command and passes something like td=touch 2010-09-17_21-54.txt to the alias command. You need to protect the special characters in the alias definition from expansion. The easiest way is to use single quotes instead of double quotes:

alias td='touch `date "+%Y-%m-%d_%H-%M"`.txt'

Then td is an alias for touch `date "+%Y-%m-%d_%H-%M"`.txt as desired.

Although it's not an issue here, I recommend using $(…) instead of `…`, so as to avoid difficulties with complex commands (backquotes have arcane and nonportable quoting rules, whereas dollar-parenthesis works intuitively):

alias td='touch $(date "+%Y-%m-%d_%H-%M").txt'

The accepted answer is perfect almost always. For those times when you MUST use double-quotes instead of single-quotes, the following works:

alias td="touch \`date \"+%Y-%m-%d_%H-%M\"\`.txt"