Your best bet is to check the O2 website? Or the modem manufacturer?
From what I've read, the Mini 9 that ships in Ireland (*) has a built in 3G modem (including simcard slot) but I think it only supports Vodafones 3G - maybe check that.
(*) I think this is disabled in US and some other countries.
It is not likely to be plug and play for it unfortunately but it does work. The version of linux on the Eee Pcs (Xandros) does have support for 3g usb modems built in which makes them a lot easier to to install.
Ubuntu doesn't appear to have it built in by default
There is at least one driver provided by a third party for free :
It may be a bit complex for you to install if you are not familiar with linux. There are also drivers provided by vodafone for these usb modems which are a bit easier to install but I found them a bit buggy on the Eee Pcs. They may work fine with Ubuntu.
It may also depend on which modem you have from O2. The older E220 which was usually white or the newer black one the E270. The driver above may work for both but I can't say for definite not having tried it.
Dell were supposed to be doing a deal with Vodafone to build in 3g/HSDPA into the Mini 9 series but I haven't heard any more about that. It may also have been UK only.
On another note the Mini 9 is not really great value here as opposed to the US. I would look at the Eee PC 901, Eee PC 1000/1000h or any of the variants (MSI Wind, Medion akoya and so on). Each to their own though.
There is a thread over on boards.ie discussing the netbooks and their relative strengths in more detail.
Dell were supposed to be doing a deal with Vodafone to build in 3g/HSDPA into the Mini 9 series but I haven't heard any more about that. It may also have been UK only.
Quick question has i have this working on my Ubuntu Laptop and it works very well.
The newest version of ubuntu has the feature built in now or if you don't want to upgrade there is the http://umtsmon.sourceforge.net/ program which is what i use.
I use the download program instead of the built in program within ubuntu because Emtsmon can sent txt messages and can read signal strength in the area. Any problems with it let me know.