Update Oct 09, 2012: In contrast to previous versions of iCal, the search function on OSX 10.8’s Calendar.app seems to be appropriately differentiating between “hashtag” and “#hashtag.” Nice! —n8henrie Tue Oct 9 17:41:13 MDT 2012
A while back, I started using hashtags in the titles of my Google calendar / iCal events (frequently created with QuickCal, which I highly recommend). I use the same thing with Simplenote / Notational Velocity.
The idea behind this is that I can use the apps' search functions to get a list of events related to a given project or topic. For better or worse, the hashtag doesn't seem to make a difference on the search results in Google Calendar or iCal (IIRC — I need to double check iCal). It does, however, make a different in spotlight searching on iOS. Compare these screenshots. One shows me all of my calendar events that I've marked as relevant to my Ob/Gyn rotation, the other shows me all kinds of stuff that happens to have the letters Ob.
The difference is even more important in Simplenote / Notational Velocity, where "#temp" efficiently pulls up all of my temporary notes, as compared to "temp", which would give me every note that contained the word template, temporary, temper, contempt, etc.