iud_divas have an IUDs and menstrual cups (http://iud-divas.livejournal.com/tag/iuds%20and%20menstrual%20cups) tag that might be a useful resource for you in addition to the answers you get here :-)
The summary seems to be that after the first few weeks with an IUD there is no particular risk involved in using a cup. Certainly I used a cup (Mooncup UK) while I had an IUS (Mirena) without problems; I did take extra care to always break the seal thoroughly before removing the cup, and I did check my strings regularly, but that was all. Anecdotal evidence seems to be strongly in favour of "it's fine".
no subject
The summary seems to be that after the first few weeks with an IUD there is no particular risk involved in using a cup. Certainly I used a cup (Mooncup UK) while I had an IUS (Mirena) without problems; I did take extra care to always break the seal thoroughly before removing the cup, and I did check my strings regularly, but that was all. Anecdotal evidence seems to be strongly in favour of "it's fine".