You're fine. The danger was having the firmware upload be interrupted or corrupted during the minute or so that it takes to upload. Any interference from other wireless users could easily crash the firmware upload resulting in comatose router. However, even if this happened, there are instructions for "de-bricking" a WRT54G on various web sites. Consider yourself lucky and don't do that again.
Plug: Look into alternative firmware for the WRT54G such as: