Gary wrote earlier that part of the new Yahoo Site Explorer (placeholder page for now) service to come is a new bulk submit option for Yahoo. While we still wait for Site Explorer, Barry Schwartz at Search Engine Roundtable reports that the bulk submit part is now live.
It's rudimentary compared to Google Sitemaps, in that you can't prioritize pages, ping Google that you have updates or anything like that. On the other hand, there's a big, big plus in the simplicity. Just make a text file with a list of your URLs, one URL per line. Then submit the location of that file via this page. Have fun!
By the way, Google Sitemaps will also accept a text file in the same manner. So if you've done one for Google, you're set for Yahoo. Doing one for Yahoo? Then you're OK for Google.
FYI, for the "what's old is new" set, this is exactly how Infoseek worked back in 1997, except for the instant inclusion. When you gave Infoseek your list, all the URLs got in. Yahoo's service, like Google's, is merely a way to suggest that pages get crawled and added. There's no guarantee they will.