Jim,
First, let's make one thing clear - Erik just sketched the procedure for LPCs (based on his prior experience with RD2/66x), but it needs a bit more - so I hope you are referring to
http://www.esacademy.com/software/flashmagic/resources/touchless935.pdf (by Lex Timmerman, based on Erik's work). Erik mentioned this, too.
1. Place the routine where it suits to you. Placing it in an ISR activated by pushbutton is a good idea if you have a spare input; Erik's remark (activate it by holding down the Rx pin = break) aims to spare this pin (alternatively you can enable the bootloader activation by break as Andrew said earlier). If you have some communication protocol established, maybe it's also a good idea to implement a command which results in call to this routine, so you can get into bootloader without opening the box (again (C) Erik).
If you insert it on 0000h, upon first running your application it gets activated and the subsequent reset will invoke
2. Not absolutely completely - it is maybe also a good idea to include restoration of the boot vector (its exact value depends on amount of FLASH on that particular LPC9xx), just for the case you accidentally change it during ISP.
3. You can also embed the whole bootloader or its part into your application. There is an appnote at Philips site, including the bootloader's source code in the accompanying zipfile. You can at least get inspired by it...
Jan