There are a few things that you need in order to have 3G on a Mini9:
1) WWAN card slot (to hold the Mobile Broadband card).
2) SIM card slot (if the WAN card needs a SIM in order to operate)
3) WWAN Antennas
The only Mini9's that have 3G built in come bundled with WinXP, and will say something like "Mobile Broadband" during the configuration process. These have items 1,2,3 and come with an AT&T SIM card. Last I checked they do not need to be activated by AT&T for purchase.
If you buy online from the Dell outlet store, you can usually find a 3G enabled Mini9 for ~$420 (give or take, depending on the rest of its configuration). These will show "Dell Wireless AT&T built-in cellular mobile broadband (HSPA 7.2);Wireless 802.11g Mini Card" under the "Network card" Column. These also have items 1,2,3 and come with an AT&T SIM card (not active).
You can also find a some Mini9's on the outlet store that are listed as having a WAN card slot (under the "Hardware Upgrade" section). Those models have the WAN card slot header, so you can install a WWAN card after market (you can usually buy them on Ebay or the For Sale section here). I assume they also have the SIM card slot on the motherboard, since there are only two revisions of the motherboard (those with the WWAN card header and SIM slot, and those without both). I do not know if they necessarily come with the WWAN antennas. Installing the Antennas after purchase (if you were able to get the part from dell), requires disassembling the whole Mini9 to replace the lid cover (where the antennas are installed), or installing some third party modification (i.e. buying a 3rd party antenna and jury rigging it
Now, about service providers.
Each provider has their own range of frequencies that they operate on (leaving aside the whole CDMA vs. GSM thing). With the built in WWAN card, you CAN install a Tmobile SIM card, but the factory WWAN card only supports the frequencies T-mobile uses for their EDGE network. It does not completely cover the frequencies that would be used for their 3G network, so if you were dead set on using them, you might want to hunt around for the "right" WWAN card. Likewise WWAN cards that work on Verizon and Sprint have also been successfully installed into the Mini9, replacing the factory card.
If you're Mini9 doesn't have a factory card, and doesn't have the WWAN card slot, SIM Card slot and WWAN Antennas, then it may be possible to install 3G after-market, but it will involve soldering, and from what I hear is suggested only for "skilled" people only (since you could easily fry your motherboard soldering the WWAN header onto it).
In this case, if you need/want 3G, I'd look into either a USB dongle, or tethering a cell-phone.