Paleogene transantarctic seaway
Barnes &amp; Hillenbrand (2010) argue that there is faunal evidence for a late-Quaternary trans-Antarctic seaway. From the abstract:


  We investigated biological evidence for past connectivity between different regions of Antarctica by comparing the composition of modern bryozoan assemblages from the continental margin around Antarctica. Surprisingly, we found most similarity between two areas which are not currently connected – the shelves of the Weddell Sea (WS) and Ross Sea (RS). We evaluated three hypotheses to explain this and conclude that bryozoans most likely dispersed through a trans-Antarctic seaway that opened in response to a WAIS collapse and connected the WS and RS shelves. These bryozoans must have survived glaciations(s) during subsequent ice ages in refuges, whereas they were wiped out in most other regions of the Antarctic shelf. After the last glacial period, bryozoan assemblages could freely disperse between many of the regions we examined (e.g. Antarctic Peninsula and South Shetland Islands), which has allowed recolonization of areas in which bryozoans had been eradicated during the last ice age. For the bryozoans on the WS and RS shelves to be more similar than those which are in close proximity means the trans-Antarctic seaway may have been as late as the last few interglacials.


There are some other folk in agreement, in particular Strugnell et al. (2012) on the topic of genetics of an Antarctic octopus (yes, really); and Vaughan et al. (2011) review available data and find strong biogeographical evidence for the existence of such a trans-Antarctic seaway, with four possible candidates and extensive discussion of when it might most recently have been open:


  The methods currently at our disposal do not allow us to constrain, with confidence, which recent interglacial saw the last open West Antarctic seaways. However, given that during the last interglacial (MIS 5e), air temperature in Antarctica was substantially warmer than present for several thousand years [Sime et al., 2009] and at the same time, ice-loss from Antarctica almost certainly contributed to higher global sea levels [Kopp et al., 2009], it would appear that MIS 5e is a strong candidate.


In summary: biogeography and octopuses say yes.