What changes have been made to bash in Lion?
Assuming a SnowLeopard or Lion installation that has the latest software updates, the version of Bash is the same. 3.2.48(1)-release.

On SnowLeopard you may have a 32bit compilation in some cases, whereas on Lion it is (by default) 64bit.

The command username="$(whoami)" is extemely unlikely to ever change it's output on bash, since doing so would incur the fast moving, brain devouring wrath of an infinite number of ravenous zombie sysadmin types. 

(By the way sysadmins are almost never undead, despite the occasional appearance to the contrary. The description was added for dramatic effect, and no offence to sysadmins was intended. I offer my heartfelt apology to any actual undead sysadmins, and their families, and I urge them to grab a shotgun immediately and barricade the bedroom door, double tap to be sure.)