At least 2 of 3 mobile operators in Montenegro are offering UMTS access through prepaid SIM cards (usable both from mobile, or PC with corresponding modem, or other peripheral). I'm more familiar with ProMonte offer, that is shortly explained here. The other operator offering Internet access under alike conditions is T-Mobile, and their offer is described here.

The SIM card for using ProMonte network could be acquired in their sales offices; there is a sale office in all major cities in Montenegro, and the list of offices, with addresses and phone numbers, is here. The card cost is 5 euros, allowing 50MB of traffic, and it could be re-filled later using vouchers of 5 euros (for additional 50MB of traffic), 10 euros (for 250MB of traffic), or 25 euros (for 2GB of traffic). The SIM card is data-only, it could not be used for voice calls. Modems to be used along with PC without UMTS peripheral built-in could be acquired in above mentioned sale offices too; I'm using Alcatel OT-X020 modem bought this way (the modem cost was 99 euros), which worked very well.

The signal coverage is very good in all cities, in rural areas not so good, which is kind of understandable given terrain configuration of the country. I was/am using one SIM card providing Internet access for my PC, and another card for my cell phone (mostly for VoIP, and for occasional e-mail, Web browsing and GPS navigation), and it all worked great so far. For PC usage, the modem could be easily made to work under Linux (am using it this way), while for Windows users modem drivers and corresponding dialer application installation should auto-start from the modem, when first attached to the PC. For cell phone usage, corresponding connection profiles are provided directly to your handset by sending corresponding SMS messages to the operator - see for details here (follow links under "Services" menu on the left side).