Many of my emails today start with "[SPAM]" such as for example:
 8K   Robert   [SPAM] Uptick stOck  1/12/2005   11:26 pm   
   8K   Boyd   [SPAM] Sharess in Play  1/12/2005   10:39 pm   
   8K   Gregg Souza   [SPAM] StOck Oppurtunities - their c|imbing 
1/12/2005   10:08 pm
   8K   Tamika   [SPAM] Intrepid Invest0r Rep0rt  1/12/2005   9:03 pm   
   8K   Gale   [SPAM] steaming StOck - it's g0ing thru the rO0f  
Does this mean that somebody had finally done something about spamming
and it is safe to delete these emails without looking at them?

You or your mail client has a spam filter on it. Here is a example.

SpamPal does not delete any messages, but
rather adds a ***SPAM**** mark to the email subject

Spam Filters are not 100% accurate so there is a risk that if you
delete them all without checking you might delete one that wasn't.

It's also possible that the tag is being added at the mail server
level.  A relative has an account through a small college and some
incoming email is tagged with [Spam?] before the regular Subject. 
This isn't done by their mail client or anything else on their PC,
it's being done by the admin guy in charge of the college's mail
server.

It's possible some non-spam will inadvertently get tagged with your
[SPAM] tag, so it wouldn't hurt to scan the Subjects and/or senders
before deleting them.

Is this your personal account or one at your job?  If at your job, you
might want to check with I.T. regarding the junk mail solution they
are using.

It also sometimes called tag and deliver - as probonopublico-ga says
its can be difficult identifying spam so instead of deleting it some
filters on mailservers are set up to tag suspect mail.  Users then
have the opportunity on there own email clients Outlook, Thunderbird,
etc. to create automatic rules that will redirect or delete any
messages with the subject [spam] to another folder.

The definition could be one of two:

1. "Internet spam is one or more unsolicited messages, sent or posted
as part of a larger collection of messages, all having substantially
identical content."

or

2. "Spiced Ham" (SPAM)

Believe it or not the term SPAM is probably derived from the
Monty-Python's classic Spam skit, in which the customers can only
order a menu item if it contains the processed meat. That is how most
Internet users feel about these types of messages, and only being able
to read newsgroup messages and email if they're willing to deal with
the garbage.

Check out http://www.detritus.org/spam/ for the true insight

Kind Regards

Chris Riley