Gmail on your Desktop
gmailblog.blogspot.com
gmailblog.blogspot.com
In my experience it seems that if To:/CC:/BCC: doesn't contain one of your addresses gmail sticks with your default rather than using the address of the mailbox it fetched the email from in the first place.
Alternatively you could run multiple Fluid apps (or whatever the equivalent is for your preferred OS).
Run firefox with an alternate profile (firefox -P other_profile --no-remote).
That'll start a separate firefox instance which has it's own cookies, etc.
That's what I use to have personal & public gmail accounts.
I have tons of my_name@random_domain emails, some serious, some not. They all forward to my gmail (and also to another IMAP server because storage is cheap) And gmail even lets you send email from different addresses! Yay!
(That and I find the gmail privacy thing confusing. Isn't this a problem for all the other webmail hosts? Most of the things I'm seeing are "they aren't doing it now, but they could", well, so could Yahoo, or any of the people in between. Email is not the world's most secure message medium...)
I'm grossly exaggerating now, but if we replace gmail with IRS, I'm sure that most people would regard the forwarding as a big breach of trust.
You say that a lot of people could read the mail while it is in transit. This is of course true, but a student running ettercap or a sysadmin grepping the mail queue would face criminal charges. By scanning inbound mail from third parties, Google is operating in a legally dark grey area.
I'm not aware of other webmail providers doing the same thing.
Regarding Google apps:
If I find out that a company uses Google apps, I would probably take my business elsewhere for the same privacy reasons.
Interesting that Google feels the need to use non-google.com domain names for handling the mail of apps customers:
dig -t mx salesforce.com
salesforce.com. 300 IN MX 100 salesforce.com.s8a1.psmtp.com.
whois psmtp.com
Google, Inc.
Perhaps I am old-fashioned, but an honest name would be e.g:
mx12345.apps.google.com.
I'm actually curious about that psmtp.com thing, I remember setting my mx records to point to something that shouted "yes, I use Google", maybe they have some separate arrangement? (I dug a bit, and my MX is: ASPMX.L.GOOGLE.COM)
So blame postini perhaps ;)
Granted when I attempt to log in to a Google application other than GMail the cookies do collide.... but that's the point of multiple web browsers :)