In a word, Minicom allows you to send commands (AT commands) through the chosen port (USB port, in your case), PC Suite Mode allows N900 to react to these commands (in Mass Storage Mode N900 will only give you access to files in MyDocs and will not react to any commands), and if you setup the Minicom on Windows correctly, you can send AT command to N900 which will tell it to send SMS (with appropriate number and text).
This is not exactly what you asked about, but it might be close enough to meet your needs. It will enable you do to the following:
You send sms from your PC, your N900, your Nokia 1100 or whatever. You can do it from an email client or from a webmail if you make the appropriate configuration.
Replies within 7 days come to an inbox that you can access from any computer, including your PC and your N900. The replies (or some replies selected according to criteria of your choice) can be forwarded to your email address at no extra cost, and/or to your mobile phone at the cost of an additional sms.
You can do this through an account with www.ipipi.com . There may be other messaging services that do similar things, although ipipi claims to be the only one that offers such a wide range of features. You can do all your sms messaging on that website, and optionally configure the account to operate from an email client or a webmail.
There is one disadvantage that I found sufficiently serious that I now seldom use this service (unless I am sending to someone outside North America - more on this later). Your sms enters the mobile networks from one of ipipi's gateway numbers. Sometimes a recipient will save this number, thinking that it is your own mobile number, and will use it after the 7-day period has finished. In such a case, the message will not reach you.
Regulations in Canada, USA and Mexico (but apparently nowhere else) prohibit networks from substituting your own number in place of the gateway number. For recipients whose mobile providers are domiciled overseas, you can configure your ipipi account so that messages appear to have originated from your own mobile number. In this case, replies do not come through ipipi but through the mobile networks in the usual way, and there is therefore no 7-day limitation. [Confusingly, ipipi refers to this feature as smsOUT on the Settings page and as iPiPiOUT in the Help Index. It is the same thing. The Help section could use some editing.]
[The Help Index is particularly misleading if you want to send sms from your email client. There are two ways of doing this. One appears in the Help Index, under the heading "ipipi email basics", as a link called "Send text messages from your favorite email client". This links to instructions for a configuration that conflicts with the SMTP requirements of some ISP's and with the "Subject" field requirements of some email clients. I could not make it work on my ISP.
A simpler and reliable alternative is the WebText feature listed under the heading "ipipi enhanced features", one paragraph lower.]
http://en.wikipedia.org/wiki/Minicom
I don't know how to use it. You will have to learn it.
In a word, Minicom allows you to send commands (AT commands) through the chosen port (USB port, in your case), PC Suite Mode allows N900 to react to these commands (in Mass Storage Mode N900 will only give you access to files in MyDocs and will not react to any commands), and if you setup the Minicom on Windows correctly, you can send AT command to N900 which will tell it to send SMS (with appropriate number and text).
Something about N900 AT commands:
http://wiki.maemo.org/N900_Hardware_Phone