I haven't found a similar button.. But there are many things that have greatly improved alarms in my opinion:
1) You can now add as many alarms as you'd like, and they may be of different types (screen, email, file).
2) iCal need not be running for the alarm to work at an appointed time. (But I think you do need to be logged in - haven't tried it w/o being so.)
3) In the popup menu next to each alarm, you can select from previously used alarms... so this goes some ways toward making up for the lack of a default alarm option - but it does require an extra selection.
4) iCal now has the built-in, default ability to open applications and files upon the occurrence of an alarm. Used to be you could enable iCal to run an applsecript, but you couldn't rely on other people's having this functionality in their install.
5) The setting of alarms is now scriptable. The upshot of this is that other programs can set events in iCal with alarms. So, for example, programs like iCal calling itunes need not be running all the time - iCal can start them. There are, obviously, many other possibilities with this new feature.
None of this is to say that a default alarm wouldn't be nice, but I do think that 1.5.1 is, overall, a *big* step forward with respect to alarms.