The Collaborative International Dictionary of English v.0.48:

check in \check` in"\ (ch[e^]k`[i^]n"), v. i.
   To register as a guest at a hotel, inn, motel. etc. Converse
   of check out.
   [PJC]

WordNet (r) 3.0 (2006):

check in
    v 1: announce one's arrival, e.g. at hotels or airports [syn:
         check in, sign in] [ant: check out]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "check in":
   accomplish, achieve, approach, arrive, arrive at, arrive in,
   attain, attain to, be received, blow in, bob up, book, buy it,
   calendar, carve, cash in, catalog, chalk, chalk up, check out,
   chronicle, clock in, come, come in, come to, come to hand, croak,
   cut, docket, drop off, engrave, enroll, enscroll, enter, fetch,
   fetch up at, file, fill out, find, gain, get in, get there, get to,
   go west, grave, have it, hit, hit town, impanel, incise, index,
   inscribe, insert, jot down, kick in, kick off, kick the bucket,
   knock off, list, log, make, make a memorandum, make a note,
   make an entry, make it, make out, mark down, matriculate, minute,
   note, note down, pass out, peg out, pipe off, place upon record,
   poll, pop off, pop up, post, post up, pull in, punch in, put down,
   put in writing, put on paper, put on tape, reach, record,
   reduce to writing, register, ring in, roll in, set down, shove off,
   show up, sign in, step off, tabulate, take down, tape, tape-record,
   time in, turn up, videotape, write, write down, write in,
   write out, write up