R5RS provides a simple mechanism for easy cases of lazy evaluation. It does not support generalized lazy evaluation, because all built-in procedures are eager whether they 'need' to be or not. The relevant identifiers are delay and force; they are not present in IEEE Scheme. SRFI 45 argues that this pair is insufficient for expressing common lazy algorithms in a way that preserves tail recursion, and adds lazy (equivalent to (delay (force ...)), but acting atomically) and eager. The semantics of delay and force remain downward compatible.
Regarding the sample implementation from R5RS, it does memoize via make-promise so there's no need to change it.
WG1 voted to reject SRFI 55.
I favor the SRFI 45 argument; see the SRFI itself for details.
I think the four identifiers should be in an optional module.
Editorial note: the sample implementation given in R4RS and R5RS does not conform to the specification (it doesn't memoize) and should be removed or replaced.