Insert lines between delimiters

I'm working with a file like:

somestuff
somemorestuff
...
someadditionalstuff
STARTTAG
ENDTAG
someotherstuff
somecoolstuff
...
somefinalstuff

I've got some text (either in a file or piped) to put between STARTTAG and ENDTAG. I was thinking something like grepping for the line number of STARTTAG and ENDTAG and doing appropriate head and tail and so forth, but it'd get ugly by the time I was done. Is there a cleaner/easier way? I know how to do it in Perl, but surely there's an easier/faster/better option.

Thanks!,
BMDan

If sed is allowed:

sed '/STARTTAG/r insertfile' file > newfile

Regards

Always with the neat sed tricks! /me files that one away.

Thanks,
Dan